Once removed, use a 9V-block to replace the original battery. The "-"-contact should be the one in the front, facing you. This is the bigger contact marked with a red circle.

Since you already have a label printer on hand, it's a good idea to stick this polarity information next to the compartment for future reference. E. g. "'-' in front"

Since the 9V-block is smaller than the original battery, wrap it in some bubble wrap or similar. It is also shorter, so add some cushioning to the top of the cavity.

The original won't charge, so replacing it with an OEM replacement is likely futile.