First of all, you should know what is minor account and what is major account. Minor account is the account created in any bank when you are under aged. That is below 18 years. For example, during your school studies or college studies, you may have opened student account for scholarship check, loan, etc. That student account is minor account. So the major account is the account created after 18 years of your age or older. If you have minor account, you can't get complete features like major account. So after crossing the age 18 or above, you should the close your earlier minor account or just convert it to major account. In this connection, you should approach your bank manager and submit a request letter to change your account from minor to major. You need to submit age proof along with your application. Here's a sample letter to write a request letter for this purpose.
I have opened an account when I was student at the age of 16. Now I crosses 18 years old. So I wish to use my account with all features by converting it to major account. Please kindly do needful. I have attached my account details and proof of age.
How to write letter to bank for converting minor account to major account- Sample Letter Format
55, Municipal Street,
The Bank Manager,
Respected Sir / Madam,
Sub: Request letter to change minor account to major account
My account number: _________________
1. Xerox copy of bank passbook
2. Proof of age, Aadhaar card. [You may use any other proof too]
Note: In some places, you need to give your information to make this letter meaningful. The details here shown are for writing this letter format only. Those details do not belong to anybody.