If it will not quite shift up to the next ring, turn the barrel adjuster, that is the knob right where the cable comes out of the shifter, like you were trying to unscrew it, 1/4 turn. Test the bike, if you still have hesitation then give it another 1/4 turn. If this does not take care of the problem then it is probably a height or angle problem and will need bike mechanic attention for sure.
And of course, if you have free service, let them do it