We don't currently have an Undo button or a detailed changelog for your schedule. We're planning to add this sometime down the line.
The PRO version has a snapshot system that allows you to store versions of your schedule to roll back to if anything goes awry, e.g. if you've made some changes you decide you no longer want.
Click the Last snapshot saved... link at the top of the schedule maker:
This will bring up the snapshots dialog:
autosaves are created after each schedule change. if the last change is less than 15 mins ago, the latest autosave is overwritten. if the last change was over 15 minutes ago, a new version is created.
up to 10 autosave versions are kept in storage. once you reach that number, the oldest autosave will be deleted when a new one is created.
to prevent auto-deletion per the above rules, pin an autosave. you can enter a name for the autosave version when you pin it.
additionally, you can create manual snapshots at the bottom and enter a name for the version.
an autosave is created before each restoration, so you can always roll back to what you had before you restored a snapshot.
Note: if you change the underlying settings of your schedule (number of teams, games, weeks, adding playoffs etc.) this can interfere with a snapshot's ability to be restored. snapshots are mostly meant to serve as backups of your schedule itself, not so much to try entirely different setups.