I got 06 month "M" type visa. 02 entry, max stay 30 days.

Sameera, after the first 30 days run out, you should apply for the extension at the local exit and entry administration office by providing all required documents. The extension won't be more than30 days. After the extension is used up, you should exit and activate anther entry. After the 30 days end, you can try to extend once again. However, I'm afraid the immigration official will not agree with it. I personally think the M type will not allow you to stay for four months in total.