Joints are the edges, locks are the bottom of the piece. You need to start at the lock on the piece above the one you want to remove, wich would be the first piece above the window. At the lock above that piece, you use a tool called in the trade, a zip tool. You can buy it at a hardware store for cheap money. Useing that tool ,unlock the piece above the one above the window. CAREFULLY, you dont want to dent or bend the siding. Once unlocked, you can gently lift the loose piece and see the nails that attach the piece. CAREFULLY pull the nails with a flat bar or BIG catspaw. After removeing the nails, pull down on the piece comming off. It will have a cut-out to go around the trim, with a sill trim piece crimped to it. That will stop you from pulling down the piece at the top of the window. you can loosten the sides of that piece ,then lift it out. CAREFULLY, you don`t want dents. When going down the sides of the window, unlock and remove the same way, be sure to remove the whole piece, don`t let them flap freely and try to work under them. The bottom piece is not so difficult to remove,you`ll get the idea before reaching the bottom piece anyway.
Instalation is the reversal of removal, BUT, push up with your hands to lock back in the pieces, you`ll only need the zip tool for the last piece. Use it the same way you did to remove it, to lock it.
Be sure to lap them the same way they were. You can tell cuz the edges will look new under the lap.
Good luck and take your time !!