To replace sidelight glass, pry off the molding that holds the glass in place, remove the glass, replace the old silicon caulk with a new one, install a new glass, and reinstall the molding. Examine the sidelight in advance to determine the side that can come out easily.
Plunge the tip of a putty knife between the glass and molding, which can be on either or both sides of the glass, and loosen the molding by moving and tapping the knife gently. Repeat the process on all sides of the panel. Move the molding from side to side gently with your fingers until it breaks loose from nails, and use a pair of diagonal pliers to pry off the nails.
If the glass doesn't come out on its own, insert the edge of the putty knife between the glass and the frame, and move the glass forward with less force. Remove the glass sidelight, and set it aside. After removing the old silicon, place the new bead of silicon along the corner of the frame, install a new sidelight, and press the glass to flatten the silicon and attach the glass.
Install back the moldings in the same manner they came out, use a small block of wood to slightly tap the corners, and insert nails into the molding at 30-degree angles. Be sure to leave 4-inch spaces between the nails.