Originally Posted by
soyabean
On some of these bikes there is no cable release lever, so you have to deflate the tyre to get the wheel out.
I've found that removing one shoe from the caliper will allow wheel installation and removal without deflating the tire.
To get to the OP's apparent issue, to remove the pad itself from the holder, you can often pry the pad out with a screwdriver. If the holder is of the type that is stamped from sheet metal, with the edges folded up to retain the pad, you can bend one end of the holder down to create an opening through which you can drive out the pad and slide in a replacement. N.B. Most of these types of holder are aluminum, and can be weakened by bending the tab to create the opening; after doing so, position the pad with the weakened tab toward the rear, so that braking action does not bear against the weakened tab.
Another option is to replace the entire pad/holder assembly; these are usually quite generic. The Dia-Compe "G" caliper in your picture uses a 6mm x 1mm (M6) threaded stud to secure the holder to the caliper; this is a very common design, and any replacement with the M6 stud ought to work.