About M visa

Hi, i have M type: one year, multiple entry, max stay 30 days. i'm staying in xiamen city and i'm planing to stay two or three months more, so should i go back my country and re-enter china again or i heard that i can do it in shenzhen just need to cross the gates between shenzhen and hong kong and re-enter again. and after re-enter should need to register the local police station. or not need this for that M type . thanks in advance