There are several changes to cards occurring with the release of Dominaria United. You can find the official text of any card on Gatherer.

Yotia Declares War

Yotia Declares War

Yotia Declares War was mistakenly printed with the word "artifact" omitted from its final chapter ability, and it has received an update to correct that.

Yotia Declares War's old text:
Read ahead (Choose a chapter and start with that many lore counters. Add one after your draw step. Skipped chapters don't trigger. Sacrifice after III.)
I — Create a 0/2 colorless Thopter artifact creature token with flying named Ornithopter.
II — Tap any number of untapped artifacts you control. When you do, Yotia Declares War deals that much damage to target creature or planeswalker.
III — Up to one target artifact you control becomes a creature with base power and toughness 4/4 until end of turn.

Yotia Declares War's new text:
Read ahead (Choose a chapter and start with that many lore counters. Add one after your draw step. Skipped chapters don't trigger. Sacrifice after III.)
I — Create a 0/2 colorless Thopter artifact creature token with flying named Ornithopter.
II — Tap any number of untapped artifacts you control. When you do, Yotia Declares War deals that much damage to target creature or planeswalker.
III — Up to one target artifact you control becomes an artifact creature with base power and toughness 4/4 until end of turn.

Out of Time

This card was printed with a nonstandard template that instructed a player to phase permanents out instead of saying that the permanents themselves phase out. This has now been corrected.

Out of Time's old text:
When Out of Time enters the battlefield, untap all creatures, then phase them out until Out of Time leaves the battlefield. Put a time counter on Out of Time for each creature that phased out this way.
Vanishing (At the beginning of your upkeep, remove a time counter from this enchantment. When the last is removed, sacrifice it.)

Out of Time's new text:
When Out of Time enters the battlefield, untap all creatures, then those creatures phase out until Out of Time leaves the battlefield. Put a time counter on Out of Time for each creature that phased out this way.
Vanishing (At the beginning of your upkeep, remove a time counter from this enchantment. When the last is removed, sacrifice it.)

Cleric Class

Cleric Class's last ability returns a creature card to the battlefield and causes its controller to gain life equal to its toughness. The word "its" there is a little ambiguous in the case where the creature card in the graveyard had a toughness different from the creature entering the battlefield (such as when returning a Clone to the battlefield). The intent of this effect was to refer to the creature on the battlefield, not the creature card in the graveyard, to determine how much life its controller gains. However, it turns out that "it" has been used a little bit inconsistently on printed cards, so this led to understandable confusion about the effect. We've updated the Oracle text of Cleric Class to refer to the creature explicitly.

Cleric Class's old text:
(Gain the next level as a sorcery to add its ability.)
If you would gain life, you gain that much life plus 1 instead.
{o3oW}: Level 2
//Level_2//
Whenever you gain life, put a +1/+1 counter on target creature you control.
{o4oW}: Level 3
//Level_3//
When this Class becomes level 3, return target creature card from your graveyard to the battlefield. You gain life equal to its toughness.

Cleric Class's new text:
(Gain the next level as a sorcery to add its ability.)
If you would gain life, you gain that much life plus 1 instead.
{o3oW}: Level 2
//Level_2//
Whenever you gain life, put a +1/+1 counter on target creature you control.
{o4oW}: Level 3
//Level_3//
When this Class becomes level 3, return target creature card from your graveyard to the battlefield. You gain life equal to that creature's toughness.

Delina, Wild Mage

Delina, Wild Mage creates tokens that have a very normal-looking delayed triggered ability—except it isn't technically a delayed triggered ability at all! It's a regular triggered ability that uses the template we normally use for delayed triggered abilities. The wording has been updated slightly to put the trigger condition of that ability at the front of the ability.

Delina, Wild Mage's old text:
Whenever Delina, Wild Mage attacks, choose target creature you control, then roll a d20.
1–14 | Create a tapped and attacking token that's a copy of that creature, except it's not legendary and it has "Exile this creature at end of combat."
15–20 | Create one of those tokens. You may roll again.

Delina, Wild Mage's new text:
Whenever Delina, Wild Mage attacks, choose target creature you control, then roll a d20.
1–14 | Create a tapped and attacking token that's a copy of that creature, except it's not legendary and it has "At end of combat, exile this creature."
15–20 | Create one of those tokens. You may roll again.

Fatal Grudge

Fatal Grudge's text refers to a permanent's card type but was missing the word "card" in its text.

Fatal Grudge's old text:
As an additional cost to cast this spell, sacrifice a nonland permanent.
Each opponent chooses a permanent they control that shares a type with the sacrificed permanent and sacrifices it.
Draw a card.

Fatal Grudge's new text:
As an additional cost to cast this spell, sacrifice a nonland permanent.
Each opponent chooses a permanent they control that shares a card type with the sacrificed permanent and sacrifices it.
Draw a card.

Introduction
Comprehensive Rules Changes
Oracle Changes