Your first step is to check - are you eligible for a refund?

• Purchased on Steam: Stormgate’s refunds are done through Steam and are subject to Steam’s DLC refund policy which you can find here on Steam. It states that DLC refunds are permissible “within fourteen days of purchase, and if the underlying title has been played for less than 2 hours since the DLC was purchased, so long as the DLC has not been consumed, modified, or transferred.” (Quotation obtained on July 29, 2024). This does apply to pre-purchases. 
• Purchased in-game: you may elect to self serve a refund on Steam within 48 hours as long as the item has not been consumed, modified, or transferred.

If you are eligible - how do you submit a refund?

Refunds are done through Steam. Follow their documented process here.