How can I move subscrition to my other email address?

I've been using my 2 fingboxes quite happily - they are linked to emailaddress_A.
Some time ago I signed up for free Fing Desktop using emailaddress_B, but could never see the Fingboxes on it.  Today I signed up for the premium account using emailaddress_B - at which point I realised I'd done it on the wrong account.
How can I transfer the subscription onto emailaddress_A?