Primary driven gear tutorial/question

Hi! My PK50XL2 has a 75cc cylinder and I now want to change the primary driven gear to get longer gearing. Do anyone have a step by step guide on how to do this? I'm new to Vespa and I'm not sure how to proceed. I understand that the primary driven gear can be changed without dropping/splitting the engine, but it would be awesome if someone could explain the procedure of changing it :)