First you can go on the existing system to edit app and click rollback, then select the version to roll back to. Their will usually be several versions if the app has been installed awhile and is is the easiest way to roll back an app.
Another way is if you go to Data Protection >> Periodic Snapshots and click the external icon It will open the full Periodic Snapshots page. On the top right, Click on the button named Snapshots. This will open the full Snapshots page will list all of your snapshots on the system starting with the first 10. Adjust the number to view per page if desired. Snapshots will have a path ending in the app something like this: my-pool/ix-applications/releases/my-app for each installed app.
You should be able to scroll down the list and find a snapshot from the Frigate 0.13 install before the date of the update to 0.14. Open the dropdown on the desired snapshot and click roll back. These are automatic snapshots taken each time an update is made to the app and don’t expire.
The app store install only provides the latest version of the app and there is no way to select an earlier version to install. So on a completely new install of Truenas or a completely new install of an app there will not be any previous snapshots for the app to roll back to.