Rules for Updates
When a card is too strong, too weak, or should otherwise be changed, you can submit an [Update] to the Subreddit with a modified version of the card.
The top 3 voted update submissions with more votes than the 10th normal submission each week are accepted. (The max 3 updates have been nicknamed "update slots" by the community.)
Rules for Updating Cards
You can update anything about a card, including its cost, what it does, its art, etc.
The purpose of an [Update] is to rebalance a card, not redesign it. We believe that no card has "inherently problematic design," and all can be rebalanced without completely redesigning it. Updates are necessary for the game's health, but should never be mean-spirited. This has some follow up points:
1. No 'ground up' redesigns
The definition of a "ground up redesign" is ambiguous, and hard to define, but if you change the original idea of the card too much, we may not allow your submission. (This doesn't look like it will come up very often at all; we still have yet to see any serious submissions which go this far in the Subreddit.)
2. You cannot remove a card
You cannot change a card which had a rarity to be 'undraftable' rarity (effectively removing it from constructed play), or give it an intentionally absurd casting cost like 99 mana. This is an occasional desire from players unhappy with a card due to a high power level; however, it is our (the developers) opinion that any card can be changed to be balanced by changing its casting cost or adding prerequisites for its effects to occur.
3. Keep the creator name the same
This follows logically from #1.
4. If you changed the art, you can change the artist name
Your change should be a significant and noticeable change to the art.
(If this is the only thing you have changed, you can submit it as a [Cosmetic Update] instead.)
5. No changing a card to remove its Design Competition restriction
If the card you're updating was the winner of a Design Competition with a specific restriction, your update cannot change the card to break the restriction.
6. Cards can be updated the week they are added
[Update]s or [Cosmetic Update]s to cards which have not been added to the game yet are allowed; if the [Card] post gets in and the [Update] or [Cosmetic Update] does too, it will be updated before it is added to the game. This has not been utilized as far as we (the developers) can tell.
7. Core Set cards can’t be updated
For more information, you can read a detailed explanation here.