Can we change registered Mobile no. without logged in to Quick books? if that is Possible what is the method to do so?

Yes, you can, ceo16.

I'll be glad to walk you through the steps.  Here's how:

1. Go to this link: https://accounts.intuit.com/index.html
2. Select Sign-in & security on the left panel.
3. Click the Phone menu and choose Change beside the number.
4. From the drop-down, select the Flag icon and choose your country. QuickBooks will automatically select the country code.
5. Enter the phone number and password.
6. Press Save.

You can also refer to this link for additional reference: Change your email, user ID, or password for QuickBooks Online, QuickBooks Online Accountant.

You can always get back to this thread if you have additional questions about managing your account. I'll be around to help.