I mistakenly added an extra seat and would like to cancel it.

I have been using a full seat on a monthly plan, but today I accidentally added 1 full seat. while trying to find the menu to cancel my monthly subscription. The invoice has already been updated to include this change, and I wanted to cancel the extra full seat right away, but I cannot find any option to do so. This just happened today so I need it sorted today so there is no claim that it has been used. I have removed the collaborator to a view seat