Hi.
By changing the gearbox....simple............or you can check that the kickdown cable is working, tis next to the throttle cable, get someone (without the engine running) to operate the throttle and see if it moves, if it does, extend it for slower response to kickdown (more pedal movement).
Mine stuck, left it on car and gave it a good dose of wd40 all fixed, much cheaper than a new gearbox.
Cheers, Nick.