I have accidentally purchased 1-year subscription

The following has instructions on how to make a refund request: Request a refund for apps or content that you bought from Apple - Apple Support

It actually depends on when did you purchased the product. I have not done it yet but this is the instructions provided by apple:

1. Go to reportaproblem.apple.com.
2. Sign in with your Apple ID and password.
3. If you see the "Report" or "Report a Problem" button next to the item that you want to request a refund for, click it.
4. Follow the instructions on the page to choose the reason why you want a refund and submit your request.