An application for change mobile number in bank account is a formal letter written to the bank manager when you want to update the mobile number linked to your account. This is needed for SMS alerts, OTP, and net banking security.
Below are simple sample formats you can edit and submit at the branch.

Sample 1: Change Mobile Number Request
To,
The Branch Manager
[Bank Name]
[Branch Address]
Date: [Write Date]
Subject: Application for change of mobile number in bank account
Respected Sir/Madam,
I am an account holder in your branch with Account Number [Account Number]. I am no longer using my old mobile number and request you to update my registered mobile number as given below.
Old mobile number: [Old Number]
New mobile number: [New Number]
Kindly update the same in your records. I have attached the required documents.
Yours faithfully,
[Your Name]
Address: [Your Address]
Mobile: [New Number]
Signature
Sample 2: Simple Mobile Number Update
To,
The Manager
[Bank Name & Branch]
Date: [Write Date]
Subject: Request to change mobile number linked to bank account
Respected Sir/Madam,
I hold a savings/current account number [Account Number] in your branch. Please change my registered mobile number from [Old Number] to [New Number].
All necessary documents are attached. Kindly do the needful.
Thanking you,
Yours faithfully,
[Your Name]
Signature
Sample 3: Mobile Number Change with Account Details
To,
The Branch Manager
[Bank Name]
Date: [Write Date]
Subject: Application for updating mobile number
Respected Sir/Madam,
Account Holder Name: [Your Name]
Account Number: [Number]
Old Mobile Number: [Old]
New Mobile Number: [New]
Please update my mobile number in your records so that I can receive OTPs and transaction alerts on the new number.
Yours faithfully,
[Your Name]
Signature
These samples for an application for change mobile number in bank account can be used at the bank branch along with ID proof.