SA answer: Start by removing all the components & wheels, then upgrade the frame, wheels and components and you will be good to go.

Real answer: break-down and rebuild it, inspecting everything as you go. I suggest you pick a chainring and stick with it. Pick the middle ring and remove the front derailleur. If it were me I'd turn it into a single speed to avoid rear shifting problems also, but my ride is flat. Most importantly though, realize that the bike was probably assembled by a 16 year old kid with know clue how to put a bike together properly, so you need to double check everything before you start to rely on it for transport.