Refunds must be requested via the Store from which you originally made the purchase, and can only be made in accordance with that Store's refund policy.
If you made the purchase via the Steam Store then please visit:
https://store.steampowered.com/steam_refunds/
Please do not try to initiate a refund in any other way, as doing so may result in your account having negative credit. Negative credit will prevent you from using your account.