1) Screw out the 2 x screws and the cover at the side of the main door
2) Screw a little inside the '-' screw which is protected with a 'C' circle in order to open the lock of the Key-Cylinder.
3) Insert the Old Bonco key (Half not 100% inserted) into the Key-Cylinder as a handler for screwing out.
* Put your new Key-Cylinder and screw inside and reverse the procedure for the installation.
Good luck All.
** I have installed a new Key-Cylinder at Saturday. But my one is not Bonco one so other Key-Cylinder would be fine for RC too.