If you've received a refund for a business expense, then you'll need to log this in the app. Refunds are typically marked with a green 'Refund' tag on the transaction, so you can easily see which ones they are.
You should categorise your refund using the same category that you used for the original expense, this will ensure that the refund amount cancels the original transaction.
For example, if you buy £100 worth of materials from a shop, you'll categorise the £100 expense as 'Materials'. If you then decide to return these items and you get a refund, the refund for this would then also need to be categorised as 'Materials' giving you a net 'Materials' sum of £0.