Despite popular belief, there is indeed such a thing as stupid questions, but this is definately not one of them

If you really want to keep your current bike and upgrade it, then nothing in the world should stop you from doing so.
If you don't know how to fix bikes yourself, you'll need someone to do it for you and probably pay him to do so.
A better thing to do would be to learn about bike mechanics and upgrade it yourself.
It takes a while to get into, but it isn't that hard for anyone with basic mechanical skills.
Parts can easily and quite cheaply be ordered from all kinds of online shops and these are generally (a lot) cheaper than local stores.
Be sure to know exactly what you need before ordering though, as most parts are only compatible with certain bikes or other parts.
Good luck with it.
If you need any more help with it feel free to ask