You can change your paid MDN Plus subscription plan through your Firefox Accounts subscriptions page.
- Access your account at https://accounts.firefox.com/signin.
- Click Paid Subscriptions on the left panel to open the subscriptions page.
- Click Cancel next to your current MDN Plus account.
- Your MDN Plus subscription will revert to a free account. You can then sign up for a different plan at https://developer.mozilla.org/plus#subscribe.