If your bike has Tourney components, then normally it will be cheaper to buy a complete bike rather than replace parts like the shifters. The Tourney on your bike is most likely 7 speed (this model last had Tourney spec'd on in 2008, and was 7 speed), and upgrading to anything which will give better shifting, will require an upgrade to either 9 or 10 speed, as parts just aren't produced for the mid range in 7 speed now (there is the possibility of NOS parts but you can't just pick thee up at a bike shop).

If you are having issues with your shifting, would be looking at why; at the cables, routing and RD hanger, are all these good / aligned? before spending any money on new parts like shifters and RD's.