How do I use my apple funds in the App Store
How do I use my apple funds in the App Store to buy apps?
If you mean your Apple Store balance in the App&iTunes Store, I don't think you can. The link below is about cards but illustrates the distinction between the stores.
"To find out how to redeem your gift card and what you can buy with it, check which type of gift card you have" --> https://support.apple.com/HT204199