First, click on the Invoice ID.
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This will open a detailed view of that invoice.
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Click on the Three dots icon adjacent to the Invoice download button.
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Once you click on the Three-Dots icon, you will see a drop-down menu.
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In that menu, select "Issue Refund".
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Once you click on Issue refund, you will see a dialogue box.

In that dialogue box, you can enter the amount that you wish to refund to your customer.
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​It is your choice whether to issue a full refund or a partial refund.
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​Just enter the amount that has to be refunded and that amount will be refunded to your customer.
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Also, you can mention the reason for the refund as well on the same dialogue box.
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Finally, click 'Issue Refund'.
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That is all.
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The added amount will be sent as a refund to your customer.