How to change from 250 plan to 500 plan

now i have 250 plan broad band service, how will i change the 500 vombo plan in my business

Write an application in which the subject says 'CHANGE OF BROADBAND PLAN OF TELEPHONE NO. 123457'. After the application is written submit it in the BSNL customer care and confirm whether the plan has changed or not on the local telephone exchange no.