Field update

I have a drop down field called "Player Name". When I click on a player from the list, it brings up all the player data including the a drop down box called "Prepaid Sessions".

When a player attends a session, I want to be able to change the number of sessions on this form by picking the new number from the drop down list and having it update the record of that Player. (not add a new record of that player.) Every time I do it now, it seems to add a new record for that player. (giving me multiple entries)

Is this doable?
rwood