today is Feb 04, changing plan is very simple but only thing with changing plan is that it will be affecting only on next billing cycle, so if you give change plan request today, it will get effective from 1st March only.

So to change plan, get copy of your latest paid bill and write an application on plain paper mentioning what you want with your all detail ie. address, phone No, current plan.

Goto BSNL office nearby, ask them for broadband dept. and give it to them and do not forget to take  acknowledgment receipt.   

I changed my plan last month in same way.