How to zero my Apple ID 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 balance, click -> Get Support

2.                                                                                                      

See -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

scroll to ⤵️

If you have Apple ID balance 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 (KW)