Can I Undo a Rename?

No. Once you have clicked Perform Renames and confirmed the rename, the files carry their new names and there is no automatic way back. Edit › Undo only reverts changes you make to the settings of an action, not a completed rename.

The confirmation dialog exists for this reason: check the new names in the preview before you confirm, and scroll through the whole list for large jobs.

Getting the old names back

• Rename them back. Keep the files in the list after renaming by unticking Clear preview list after renaming in the Settings window, then apply the reverse action, e.g. remove the text you added
• Save a file list first. Before a rename you are unsure about, choose File › Save File List… and save the tab-delimited list of current and new names. To reverse the rename later, swap the two columns in a spreadsheet and use the Rename from file list action
• Use a backup. Time Machine and other backups keep the files under their old names

Renaming never changes the contents of a file, so nothing is lost even when a rename goes wrong; see also Why can’t I rename a file? if a rename was refused.