Unlock your iPhone and head to “Settings.” 

Tap your name, which is located on top of the “Settings” page. 

Click on “iTunes & App Store.” 

On the next screen, tap “Apple ID” then select “View Apple ID” and login to your Apple ID account. 

Once you are logged into your Apple ID, click on “Manage Payments.” 

Click on “Add Payment Method,” then tap on “PayPal” on the next screen.

Log into your PayPal account and confirm that you’re adding PayPal onto your Apple ID account. 

Now, when you purchase apps within Apple apps, your iPhone should default to using your PayPal account.