Don't even know if this is possible

I want to upgrade it entirely instead of buying a new one. I own a Compaq Presario C714NR. Didn't know anything about computers when I got this, so now that I need an upgrade pretty bad, it's time to decide buying a new one, or if it's possible, completely rebuild this one with new everything. If this is possible, please let me know, if not... I'll just save up for a new one(:

Thanks.