Open up the head and remove the eyemech. See the prior tutorial if you don't know how.
Use the razor to cut carefully around the edge of the eye.
After you remove the eye, some of the rubber may remain. Do your best to clean this out. If you get clean rubbers and neat sockets you will be able to switch out the eye rubbers, even without opening the head up again (through the eye holes) and they will stay in there without glue.
Now if you want fancier eye replacements, go onto blythe chip modifying or the acrylic eyeball/mech modifying tutorial.