An amateurs reply here I'm afraid but, it always seems to work for me. There are specialist tools available but to remove the beads, I insert a stiff bladed knife at one of the mitre'd corners, ie, not against the glass but into the join on the outer face of the window frame. By twisting this gently, the bead can be sprung out of it's register and by moving the tool further along the bead, the whole section can be "sprung" out. When replacing these beads, I usually put a smear of liquid soap on the locking profile, ie, the part which clips the bead into the securing ridge within the frame, and this seems to assist the re-fitting process.