This would rather depend on which Mac you are talking about....
It ranges from dead simple (G5 tower) to bloody difficult (iBook)
However: a 20 Gb drive increase is pretty marginal. Assuming this is a Powerbook or iBook, consider buying an external Firewire drive instead. It can act as a backup and an alternat boot drive, in addition to providing gobs more space than 20 Gb. If you DO remore and replace a drive, buy a Firewire enclosure to put your old drive into.
Unless you are well experienced mechanically, are patient and meticulous, and have 4 uninterrupted hours to spare, just back away slowly from this idea.
I do it in half an hour! But I have opened my iBook 700 mhz about three times this week... bit of a problem with the display, which you have to pull now before it gives a picture