If you have full editing privileges, and:
If you are still in the grace period, you can simply edit it back to its original form and the revision will completely disappear as if it never happened.
If you are past the grace period, you can roll the edit back by clicking the "rollback" link on that revision in the post's revision history.
If you don't have full editing privileges you have to manually revert the post to its original form as a new suggested edit, which will have to go through the review process again to be approved.
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