a question please help

today i called the customer service of dataone 18004241600 and told the lady that i want to change my 900 plan to 500 she told me that i am sending an application of change of plan please contact your local commercial office.now what is commercial office and what should i do after going there?what will be my telephone plan as my plan is 900 plus telephone rent is zero so after i change my plan to 500 what will be my telephone plan?how long will it take for the plan change?

# You have to make an application for change of plan that you want to do.# Commercial Office means the BSNL Telephone Exchange or Customer Care near you.# You can opt for any Telephone plan that suit you from the BSNL bFone tariff.# Generally the plan changes on the next month or billing-cycle.