If you believe that the merchant has processed an incorrect refund amount, we would recommend that you reach out to the merchant directly to query this. You can do this by visiting the merchants website and locating their contact information. Afterpay cannot override the refund amount that is processed by the merchant.
Before contacting the merchant, have a look at this article. It is possible that the refund is correct, but it may have been taken off your remaining instalments. The amount could also differ depending on whether a non-refundable shipping charge was included.