1) PLEASE MAKE SURE NOT TO OPEN THE STIRRUP WHILE CHANGING THE PAD in order to avoid any risk to ruin the exposed security mechanism!
2) Turn the stirrup upside down; use the proper screwdriver to loosen the 6 screws under the tread.
3) The pad, placed in the upper part of the tread, will come off once you remove the screws.
4) Put the new pad in the correct position, than fix it using the 6 screws and the screwdriver. Be sure that the screws are well tightened.