How can I revert a changeset?

First of all, you should generally not undo someone else's changeset without talking to them first, and giving them a chance to repair any damage they might have done themselves, or explain why what they did is actually correct. In most cases, a response is not urgent - you can afford to give the person some time to react. (Of course there are cases of obvious vandalism where this does not hold - but often what looks like vandalism can be a newbie having misunderstood something!)

The JOSM editor has a changeset revert plugin that lets you revert changes. There's also code in SVN under applications/utils/revert which you can use if you are somewhat proficient in Perl.

Reverting changesets can be tricky, and you can inadvertently break things yourself if you are inexperienced. Under no circumstances should you attempt a revert if you are not 100% sure that, should something go wrong, you can fix it again.

The wiki page 'Change Rollback' lists various options. None of them are completely straightforward though.

Although we model OpenStreetMap on wiki principles, change rollback is not as simple as with a textual wiki. Within our interlinked vector geo-data, if edits from different users start to overlap, it can be extremely awkward.

In the future, with more careful design and development work we may be able to present change rollback tools to "mainstream" users. In the meantime the wiki page lists "power user" tools, which you should not use if you are inexperienced (Ask for help instead)

For the first part don't delete other peoples work without contacting them first.

Changesets can not be "deleted" but can be reverted with another changeset. Take care that none of the objects changed has since been changed again that could lead to conflicts. If you can use JOSM probably the easiest way is to use the reverter plugin. Otherwise there are revert scripts that can revert the changeset from the command line.

If you have trouble or doubt about how to or if you should do an revert you should contact someone on IRC or a mailing list that can revert the changeset for you.

Howto on Using JOSM revert plugin, from the mailing list

1. Check from looking at the changeset that at least some objects haven't been reverted or modified already.
2. Start josm-tested.jar (in this case from 2nd September 2011)
3. Despite the prompt I didn't update JOSM or any plugins at this stage (you need to have the plug in installed of course).
4. Download some data (any data) so that the history menu entry is not greyed out.
5. Click History / Revert Changeset
   1. choose e.g. changeset 10333257
   2. Fully revert / revert
   3. Say "yes" to download objects
6. Upload data.
7. Review warnings - there's nothing iffy there, so I'll continue the upload.
8. Use changeset comment "Revert changeset 10333257 at the request of its author"
9. Close JOSM
10. Then on OSM webpage browse to original changeset and check that some of the data has been reverted.

By SomeoneElse