Reverting is the ability to return a page to its previous state. All versions of a page are stored in the history of a page, so it is easy to revert a bad edit and return the page to its previous state.
All users have the ability to revert to a previous version of a page.
One edit reversion
If you would like to undo just one edit you simply need to:
Click on the time and date of the earlier version you want to revert to.
Then when that page comes up, you'll see something like "(Revision as of 23:19 Aug 15, 2002)" below the title.
Verify that you've selected the correct version, and click to edit the page, as you would normally.
You'll get a warning, above the edit box, about editing an out-of-date revision.
Important: in the case of vandalism, take the time to make sure that you are reverting to the last version without the vandalism; there may be multiple vandal edits.
Once you are sure the revision you are looking at is good, save the page. Be sure to add the word "revert" to the edit summary (often abbreviated as "rv"). On some wikis it is common practice to mention the date/time of the version of the page that you're reverting back to in the description as well.
You should always explain why you are reverting an article. This helps the reverted person to fix whatever problem it is that you've identified and explains to other people whether they need to even view the reverted version. If your reasons for reverting are too complex to explain in the edit summary, explain it on the talk page, and add "See talk page" in the edit summary.
Revert wars -- where two or more editors revert each other's edits over and over -- are not acceptable. High-frequency reversion wars make the page history less useful, make it hard for other people to contribute, and flood recent changes and watchlists. Unless it's simple vandalism, when someone undoes a reversion you've made, you should stop and talk to them using the article talk page or their user talk page before reverting again. Assume good faith, communicate the problem you see in their edit, and you will almost always be able to find a compromise that allows editing on the article to move forward without further disruption.
What is rollback, and how do I use it?
Administrators and users with rollback rights have an extra "rollback" link on diff pages and on user contributions lists that lets them revert vandalism faster, without the extra steps of going to the page history and opening an earlier revision. The feature is especially useful in the case of a known vandal, whose edits don't need to be checked before being reverted.
Clicking on the link reverts to the last edit not authored by the user concerned. An edit summary will be added automatically stating that a reversion was made, and the edit will be marked as minor.
Onlyuse this for reverting vandalism. It should not be used to revert an edit that you merely disagree with.
If between loading the User Contributions page and pressing rollback, someone else has edited or rolled back the page already, the message Rollback failed will display with an explanation of the problem.
If the page has been created by a malicious user and nobody else has edited the page, clicking the rollback link gives the message Cannot revert edit; last contributor is only author of this article. In this case delete the page, or replace the content with something appropriate to the page name if possible.
Can I hide flood vandalism reverts from recent changes?
A "flood" occurs when a vandal makes edits to many pages very quickly in an attempt to make the recent changes page unusable, by filling it with their own nonsense edits (and the reversions which fix the damage). In these cases, admins can choose to hide vandalism from recent changes. To do this, add &bot=1 to the end of the url used to access a user's contributions. For example, http://www.wikia.com/index.php?title=Special:Contributions&target=SomePersistentVandal&bot=1. When the rollback links on the contributions list are clicked, the revert, and the original edit that you are reverting will both be hidden from the default recent changes display.
This means that they will be hidden unless you click the "bots" link on the Recent changes page to set hidebots=0. The edits are not hidden from contributions lists, page histories or watchlists. The edits remain in the database and are not removed, but they no longer flood recent changes. The aim of this feature is to reduce the annoyance factor of a flood vandal with relatively little effort. This should not be used for reverting a change you just don't like, but is meant only for massive floods of simple vandalism.