The Bolt Merchant Dashboard allows you to issue full and partial refunds. However, given that numerous contents of your Shopify store are linked to a specific order, such as inventory management, it's best practice to issue a refund from Shopify itself. Simply select the transaction from your orders and click refund. Below is an image of how to do so. This will update the transaction in Bolt as well to keep the order in sync.
If you don't mind how an order is updated in Shopify and just want to issue a customer a refund, you can use the Bolt Merchant Dashboard to do so. Simply, select the transaction that's affiliated with the given customer and select issue refund to do a partial or full refund.