How do I change the phone number on the receipt the online donor receives?

Your form is integrated with Stripe, so I think you are referring to Stripe receipt. You can edit that under your Stripe account. As far as I know, it's under Emails settings.

https://stripe.com/blog/email-receipts

https://support.stripe.com/questions/will-my-customers-be-able-see-my-phone-number-or-address