There are 2 ways: a) Go to bank branch and give one application to register or update mobile number with the bank account mentioning your 10 digit mobile number and full account number. b) Go to Union ATM with your Union bank ATM card of same account. Inside services, you will find option called, update or change mobile number. Enter the new number 2 times, and it will get updated within 2 working days. Hope this helps. Thanks.