The resolution to this is most likely a very simple adjustment that doesn't require tools.
Some amount of "drift" is actually normal and is due to the rotational forces at play inside the hub. If this effect is happening quickly (over the course of less than 10 miles), then the most likely cause is cable tension that is too loose.
New cables actually stretch a bit at first (on all bikes), and adjustments are built into the shifting system for this reason.
On the underside of the shifter you will see these two barrels-- they have ridges on them so they can be grasped by hand.
This is a trial and error adjustment... you can choose either barrel since they both have the same effect. Back one of the barrels out about 1/2 a turn to start (there is a click every 1/4 turn)....if this doesn't solve the problem, continue to back them out. Generally alternate -- first turn one 1/2 turn,then if more adjustment is required, turn the other 1/2 turn)
This should solve the problem -- please let us know if it doesn't!