Did the buyer open a return yet? If they did not, follow the directions below
Go to your seller hub for your sold items. I have linked it below.
https://www.ebay.com/sh/ord/?filter=status%3AALL_ORDERS
Click the down arrow next to the item and click "Send Refund"
Select an appropriate reason from the dropdown, put in the amount you want to refund next to the item getting the partial refund, and click Review Refund.
Review the information in the popup. Click the "Send Refund" button and the [partial] refund will be sent.
If they DID open a return and you have the opportunity to reply to it, one of your choices MAY be to give the buyer a partial refund. If the option is not available, do the instructions above and hope the buyer closes the return.