A pairing between two users is unique to a program. First, determine which program the pairing in question is associated with. Then, head to the "Manage Pairings" page located at Programs > Pairing > Manage Pairings:
Find the pairing in the Active Pairings table (you can search for the mentor or mentee's name) and click Stop Pairing.
Email the users as needed to communicate the change.