To reload a file, use the Reload File command. This loads the file from disk again, losing all undo information. It’s exactly like closing the file, then opening it again. You cannot use Undo to undo any prior changes.
To restore a file to its original state when you first opened it, use the Restore File command. This loses all changes you made since you first opened it. It does not reload the file from disk. It returns the file to the way it looked when you first opened it. You can use Undo to undo the Restore File operation.
To open an older version of your file, use the File > Open Backup File command to pick a backup file and load it. The backup file will open in a new source file window and won’t alter the current file.