Proton VPN provides a money-back guarantee for 30 days. Unless you purchased with cash or a bank transfer, you can receive a complete refund if you cancel within this time frame. Depending on how long your plan was in effect and how long you utilised the service before cancelling, you might also qualify for a prorated refund.
Go to your account settings on the Proton website, find the subscription section, and follow the cancellation prompts.