I'm in PRC, M visa expired but I need tourist entry permit to visit HK before my return to DRC?

Agape, you should first prepare the required documents including copy of your passport, the completed application form affixed with a passport-style photo, proof of your financial support, transportation arrangement, hotel reservation, your valid entry permit to mainland PRC, and your travel itinerary to HK. And then submit your application to Beijing Office of HK SAR government to apply for it.