it's pretty straight forward, just be careful not to cut yourself on the glass when remoning the old piece.
use a razor blade or utility knife and trim out as much of the old silicon as you can around the piece to be removed. Try not to get the utility knife between the pieces as you don't want to chip the good sides. Once the silicone is removed the piece should come out without a lot of hassle (you may have to remove silicone on both the inside and out)
Once the glass is out throughly clean the edges of the good pieces and make sure you get off all the old silicone.
Go to a hardware store and have them cut you a piece of glass with the same dimensions and thickness as the one you're removing (bring the old piece with you).
Get a good tube of aquarium silicone and put the new piece of glass on the tank, use the silicone on it like the rest of the tank. Wait however long the tube says for COMPLETE CURING TIME (IMO
, at least 24 hours) and fill with water and check for leaks!