Here is a basic guide to dealing with a vandalism should one find it.
Vandalism: An edit, or series of edits that purposefully makes an article useless and/or erroneous
Finding a vandalised page
These are usually fairly easy to find by going to "Wiki activity" in the upper right-hand corner and scrolling down to see if there is a large string of edited pages by an unregistered contributor or a suspicious username.
Vandalisms can be easily reverted by going to "history" at the top of a page and either hitting undo next to the vandalised edit, or go to an unvandalised version and copy the content into the current version. one of the problems with copy/pasting is that all the pictures and links will be removed and have to be re-added.
Usually an administrator will be able to find a vandal but it saves a lot of time if you report the vandals username to a currently active administrator.
Note: Don't report anyone until you are sure they have vandalized a page, and were not just mistaken.