I've mistakenly added funds to my Apple ID and need a refund

Depending on the amount, see -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

1.

If your balance is less than the cost of a single item, contact Apple Support.

2.

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.

You can't redeem or return App Store & iTunes Gift Cards, Apple Music Gift Cards, or unused store credit balances for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can request a refund by contacting Apple Support, selecting a support category and topic, and including the following information in your request detail:

• Your Apple ID.
• 
  
• The store credit refund amount.
• 
  
• Your current shipping address.

Additional information might be requested for verification purposes.