Apple id

As Eric says you could purchase something of value greater than or equal to the amount of your store credit or try to get Apple Support to zero out your account balance using this link https://getsupport.apple.com/?caller=cups&PGF=PGF63006&category_id=SC0060&symptom_id=23330

The link eventually comes from here: Change your Apple ID country or region https://support.apple.com/en-us/HT201389. This document says: If you can't complete any of the steps, can't change your country or region, or you have less store credit than the cost of a single item, contact Apple Support

You account balance? Buy something more expensive and pay the balance with a credit card or contact iTunes Support and ask them to zero balance your account.

iTunes Support – world wide