This depends on what specifically you are talking about. If it is a switch or knob then yes, it's easy. If it is the electronics that make the parts work then, it gets more complicated.
Some guitar parts are very easy to replace, while others may require a professional luthier. If all you need changed is a pot or selector switch, you can probably do it yourself easily, provided you know how to solder.
It can be kind of tough. Plus if you do not know what you are doing you good damage the instrument or weaken the quality of sound that it creates. I would take a guitar to a professional to get fixed unless I had experience. Maybe get a cheap guitar to tool around with.
That depends on the part. Replacing a tuning peg is easy so long as you have the right size. Replacing a pickup is more complex. It requires some wiring knowledge.
