As already discussed by Zxyrra, it is possible to reverse an edit entirely by performing a "rollback".
Important special case: SPAM! The one time when you absolutely should not roll back a post is if the edit turns the post into spam. In that case, do not touch the post at all but rather just flag it as spam and move on. It will be dealt with in short order, and if you edit it, your account could be tainted by the spam flag process as well. Don't risk that; it's simply not worth the benefit. For the same reason, you shouldn't edit spam posts either; just flag them and let the system do its thing. If you believe the owner of the post is legitimately not involved in spamming, but the post is still spam, flag it for moderator attention with a custom reason and point this out. If an edit somehow snuck through the review process and turned a legitimate post into spam, diamond moderators have the ability to reverse the spam flag penalty for the user, as long as we become aware of it; in such a case, a custom flag will ensure that the post is brought to the attention of the moderators, but you should also flag as spam with the system-defined flag reason. (Post owners: This is not the same thing as saying that the penalty will always be reversed. Any such posts will be dealt with on a case-by-case basis and ultimately at the discretion of the moderator handling the flag. Hopefully it will never be an issue.)
To roll back an edit, click the time indication in the "edited" block under the post in question:
This brings up the revision history of the post. Find the revision of the post that you wish to roll back to, then click rollback above it:
You need to either be the owner of the post or have the Edit Questions and Answers privilege to roll back a post to a previous revision.
If you do not have the necessary privileges, and feel that the edit makes the post substantially worse, your choices are basically to:
- Bring it up in the Worldbuilding Chat main chat room, The Factory Floor (this is probably the quickest; chat is usually pretty well populated), or
- Bring it up on Meta (this is a good idea if it's more of a policy issue), or
- Flag the post for moderator attention and (unless it is utterly obvious) describe how the edit makes the post substantially worse
Don't roll back unnecessarily if other alternative approaches are workable. Basically, only roll back a post to a previous revision if the new revision is decisively worse than the older one. If there is some particular part of the edit that you disagree with, it's better to edit further than to roll back. However, don't simply put back what was there before; instead of simply rolling back to the previous revision, whether by editing or by rollback, consider what the person who edited the post was trying to accomplish with their edit and try to maintain the spirit of the edit while making the post yet better.