Uhm, changing computer components is actually very simple...and always cheaper than buying a full PC...
Most components you buy have a simple install manual included, and if you consider some additional (common sense) rules you'll be just fine. And computers don't have complex buildups, but are really simple. Pretty much everything is connected to the Mainboard/Motherboard, and that's it....
Rule 1: Never ever use force (only if it is explicitably required, e.g. if you have to mount a generic Intel boxed cooler. Those are quite sturdy ones...)! If something doesn't fit, you do something WRONG. Really, this is 99% true for most user-induced hardware problems.
Rule 2: Don't touch any components with your sweaty hands, every bit of (salty) liquid you get there increases danger of oxidation and the probability of you damaging the component. Hold it on the edges. Best is to use gloves.
If you follow these simple rules, you can't really fuck up anything...
For some good DAU examples check this:
http://www.dau-alarm.de