Has the buyer opened a case, this can only provide a full refund. The buyer will have to close it first, and then you can offer them a partial refund.
Be awarem the buyer may not be happy to close the case, because once closed another cannot be opened for that item with eBay, and partial refunds can be promised, but not enforced.
Once case is closed proceed this way:
If you are on My eBay Selling, go right of the item sold and from More Actions select Refund this item. Be careful to then add the correct amount of refund you wish to make, or you will refund the whole amount.
If you are on the Seller Hub, find the item in Orders >> Posted and Awaiting your Feedback. Tick on the little arrow left of the item from the drop down menu which appears scroll down a little, you will see Refund this item.
One tip, before you send the partial refund, mark the item as dispatched, there is a quirk in eBay's programming if you send a partial refund first, you then cannot mark as dispatched.
You will probably need a PC / laptop and not the app or mobile browser to do this. Chrome and Firefox work best with eBay.