How hard is it to replace the computer processor on a motherboard?

You can change or upgrade your PC's processor if it is compatible with the motherboard. With some basic knowledge and care it can be fairly easy. If you have a motherboard user guide that also helps. Typically there will be a heat sink on the processor mounted on the processor socket. You need to remove this and take out the processor and then after putting the new processor you can put back the heat sink. As processors are very delicate static sensitive integrated circuits or modules make sure you use a anti-static wrist band or at least touch a metal surface before you handle the processor.

There are a number of help pages on web for this. Do some study by going through these.