I have unused Apple ID balance. I need a refund. What should I do?

Actually it can be refunded but only under very limited circumstances. Normally once funds are in Apple Account balance the only way to get them out again is to spend them.

From: Check your Apple ID balance - Check your Apple Account balance - Apple Support -->

"About unused Apple ID balance

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 for refund of redeemed gift balances, you can contact Apple Support." Read the support article for more details.

The above excerpt only mentions balances from gift cards as being eligible for refunding, and only in limited places where laws may require that. Read the support article for more details.

If you are trying to zero your balance and still have a valid credit or debit card, you may purchase an item from the App or iTunes Stores for more than your balance. The remainder will be billed to your credit card, leaving you with nothing in your account balance.

Alternatively, you could ask Apple to remove and keep the balance. Use this link Apple provides to take you directly to the people who will do this for you --> https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0998&symptom_id=20198

If you choose the second option, use the direct link I provided. It might save you some time over using the general support link to Official Apple Support because that will just eventually take you there anyway after a bit of hunting.

Alternatively give Apple a call:

Contact Apple for support and service - Apple Support

If you are asking about this because you are changing regions, take a look at this article:

Change your Apple ID country or region - Apple Support

Money on your Apple ID must be spent with Apple, it cannot be refunded