Tower allows you to undo changes in your local files in several ways:
Discard local changes: If you want to discard the changes you made in a certain file you can choose "Discard Local changes in <file name>..." (from the contextual menu in the file list). The file is then restored to the state it had in the last commit.
Revert to specific revision: If you want to revert the file back to a version from a previous commit you should choose "Revert <file name> to Revision...". You can then enter the SHA-1 hash of the commit you want the file restored from.
Discard all local changes: If you want to discard all changes you made since you last committed you can choose "Discard All Local Changes..." from the contextual menu on your local HEAD branch in the sidebar. WARNING:Be careful with this because it resets your working directory to the last committed state - all local changes will be lost.