Edit Category Slug

Hello
During testing of uploading listings I somehow created two categories and category slugs for the same thing:

air-conditioning
air-conditioning-1

The ‘air-conditioning’ category was deleted.

There are now no listings under ‘air-conditioning’ category and one listing under ‘air-conditioning-1’.
When I try to edit the slug to remove the ‘1’ and save it, it reverts back. Is there another way to change this?

Thanks
Matt