First you need to log into your Akismet account:https://akismet.com/account/
Note: Akismet uses WordPress.com user accounts for account management. In order to access your Akismet account you need to log into your WordPress.com account. If you have forgotten your WordPress.com account password you can reset it here:https://wordpress.com/wp-login.php?action=lostpassword
Once you login, you can visit the upgrade page located here. This will allow you to choose a different plan for your account.
If you have more general questions about Akismet, please contact us here.