It is often useful to rollback (undo) a revision. For example imagine Developer1 make a change for revision 2. Developer2 then makes a change for revision3. Developer1 then realised that his change for revision 2 has introduced a bug so he wants to revert his change. He cannot revert back to revision 1 because this would loose Developer2's changes in revision 3.
In this situation you right-click the revision you want to rollback and select 'Rollback Revision'. If the file is not already checked out, this will checkout the file and copy the new data to the local file. If the file is already checked out and changed then a local revision will be created first (so you can always go back to this version if necessary).
All edits will be reversed. If the same line has been changed since this revision was created then the Resolve Tool will be launched as described in the section called “Resolving a Changeset”.
You can then make further edits to the file and submit it as described in Chapter 5, Submitting Changes