I'll provide different information about updating your QuickBooks Mobile.
If you're referring to update your mobile number or your login credentials, you'll need to sign in to your account through a browser. I'd suggest going to the User Profile and make changes on the Intuit account page.
Click the Gear icon at the top right corner.
Choose the User Profile and click the Manage your intuit account link.
You'll be routed on the Intuit account page. Then, go to the Sign In & Security section.
Go to the Phone number section and click the Verify link.
Click the Change link. Then, enter the new mobile number.
However, if you're referring to update your QuickBooks mobile app, yes, you can perform this. Please follow the steps below depending on the phone you're using.
For iOS phone and iPad:
Open the App Store.
Select the Today tab, then choose your profile.
In the Available updates section, look for the QuickBooks mobile app.
For Android phones and tablet:
Open the Google Play Store.
On the homes screen, tap the Menu ☰ icon.
Select My apps & games.
In the Updates tab, look for the QuickBooks mobile app.