That bike is not worth upgrading unless you buy parts that you can use on the next bike. Things like under seat bags, tools, patch kit and spare tube. Tail and headlights and a cyclocomputer can be moved to the next. The best bang for your buck on any bike is good maintenance. Learn how to clean and lube the chain, repack wheel bearings and bottom brackets. I have an old 18 speed Huffy that runs better then when I bought it 18 years ago. The only money I ever spent on it was chain lube and bearing grease.
