You should be able to find a site that shows how with a Google search for "replace PSOne disc drive". i willsay now that we do not recommend doing this sort of thing if you are not absolutely sure of what you are doing.
Myself, i would say it would be quicker to scrap it and pick up an inexpensive PlayStation 2, which can use the PSOne games and accessories along with open yourself up to a huge library of PS2 games (many of which can be had on the cheap).