It is possible to undo changes that were made after syncing your tree. In order to do so, please open Family Tree Builder on your computer. Go to File > Restore project > Restore from a file on disk > choose the backup that you wish to restore.
By default, we currently save the last 10 backups created in Family Tree Builder, before each sync. You can change this default to a different amount by going to Tools > Options > Sync > number of backups to keep.