M visa questions from Hungary

Hello! I had all the requirements for an M type (to visit trade shows, build contacts,..), I needed one for multiple entry (because I have to travel to HongKong for some weekends), longest stay: 90 days. But I only got a two-entry 30-30 days M type, because its my first chinese entry permit. (My boss is kind of unhappy with me)
Is there a way I can extend my stay while in PRC or do I have to came back after the 60 day are up, and apply for an M type again? If so, can I still use the same invitation letter or should I ask for a new one? Is there any advice you could give me?
Thank you!