i need to reset my App Store balance to zero

If you want to clear your Apple Account balance, it is depending on the amount:

1.

If your balance is less than the cost of a single item, contact Apple Support directly, they will help you clear your Apple Account balance, click -> Get Support

Fill in the field ❯Tell us what's happening❮

2.                                                                                                      

See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

scroll to ⤵️

If you have an Apple Account balance that you can’t use

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows the refund of redeemed gift balances, you can contact Apple Support to request a refund. 

When you contact Apple, include the following information:

•               Your Apple ID.

•               The refund amount.

•               Your current delivery address.

If you then want to change Country or Region, you must comply with all regulations described here -> Change your Apple ID country or region -> Change your Apple ID country or region – Apple Support (UK)