i have a 1 year m visa with multiple entry but the DOS is 30 days

You can extend it but the extended duration won’t be more than the original stay duration. It means that you can only get 30 days at most if you apply for an extension. Of course, you can try to extend it for the second time if you want to stay longer.
You should first prepare the relevant materials: your passport, a half-length color photo, the application form, valid registration form of temporary residence, and an invitation letter from your trade partner.
Next, submit the application to the exit and entry administration of the local PSB to extend it.