If you have just finished the replacement, and you don't like the results, you can press the Undo button.
If you have started another search or closed the program, use the Undo tab to select an old replacement and undo it. Aba will restore the files from backup.
It may happen that the files were changed since replacement. If you overwrite them with the files from backup, your changes will be lost. In this case, Aba will show you the list of changed files. You can compare them with the old version and merge the changes. The checked files in this list will be overwritten with their backups.