Once you log in to your user dashboard you can change your plan using the Change Plan link on your account page.
Selecting a different plan
On the change plan page you will be allowed to change plan based on the current usage of your account. If you are on a basic account you can easily change to any plan but if you are currently on a pro account and have more than the allowed amount of signatures for the basic account you will need to delete some signatures before you can switch to the basic account.
