To replace a resistor, just desolder the one you want to remove and solder the new one in place. They must ideally be the same size. Nowadays most keyboards use SMT components (diodes, resistors, and pretty much everything else but the LEDs themselves), which are much smaller and harder to work with, especially if you have little or no experience with soldering.
Manufacturers do not usually disclose the inner working of their keyboard controllers, especially since backlight is still a discriminating feature, so we users are left to determine how a specific backlight works by reverse-engeneering it (identifying the components on the pcb, working out how they are connected and such).