If your account status is active, you can renew you plan earlier.
1. Login to your account and check account status = Active.
2. Click on [plan and add-ons] tab
3. Click on change plan button
If you account is suspended, send a private message to modertor for help.
I think you can only do it yourself using selfserve if you are changing plans? I don't think it lets you renew the same plan without going through the Moderator Team.