Log in at www.acorn.tv then click My Acorn TV, then Manage Account, then Cancel and complete the process of cancelling your monthly membership. Then click here to go to the purchase page (make sure you're signed in to your account), add the annual membership to your cart, and then complete the checkout process.

When you return to Acorn TV, visit Manage Account. You should see "Annual Membership" and your new expiration date on that page.