Once you're logged in to your Shopify Admin, open Recheck. Then click on Order Editing in the left navigation menu.
Click on Refund methods in the Settings section
Enable the refund options you want to allow for your customers.
Refund configuration
Original payment method - The customer will get the refund back to their source payment method which they had used originally to checkout.
Store credit - The customer will be issued store credit, regardless of the original payment method. Note that you have to be using Shopify's native store credit functionality in order to use this option.
Both options are enabled - The customer will be given a choice to pick which refund method they prefer.
Both options are disabled - Recheck will not issue any refunds. You will have to refund your customers manually.