Can I edit sheet music on the Wikifonia website?

For the moment, you can't. But as a registered user you can very easily replace your own submitted sheet music with corrected MusicXML file by clicking the edit button next to your sheet. When you're not the submitter of a sheet, you can still help the other users of wikifonia by commenting below the sheet music.
In the near future, you will be able to change any sheet you find on Wikifonia by uploading a new version. By collaboration, sheet music on Wikifonia can be corrected.