It depends. Banks generally cannot increase a fixed interest rate on an existing balance, but they can raise the interest rate if any of the following conditions are true:
The rate is variable and tied to an index. If the index goes up, the interest rate will also increase.
The rate was an introductory interest rate that had been in effect for at least six months and has expired. The length of the introductory period and the annual percentage rate that would apply after the expiration of the introductory rate must have been clearly disclosed in writing.
You are more than 60 days late in making a required payment.
Review your account agreement for policies specific to your bank and your account.