How can I change my mobile number for existing subscription?

I can help you with updating the mobile number associated with your QuickBooks Online (QBO) account, @phebabasil-gmail.

You can go to your profile menu to update the mobile number associated with your existing subscription. 

Follow the steps below:

1. Go to the profile icon. 
2. Click the Manage your Intuit Account link. 
3. Select the Sign in & security menu and go to the Phone section. 
4. Click the Change link beside your mobile number. 
5. Update your mobile number, then click on Save.

You can check this article for more information in managing your account information: Update billing, payment, and subscription info in QuickBooks Online.

The following reference will guide you in updating your subscription billing information and details about your subscription: How to update your subscription and billing details. 

I’ll be around if you have any other concerns about updating your mobile number. Just let me know by leaving a reply below and I’ll answer them for you. Have a great day!