To change your Account / Billing level ie how many projects you
can use and much you pay...
Go to your Account Settings (the small link at the top right of
your screen) Click on that.
Then the first section you an see is "Account Limits". This shows
you your current Account level.
To Upgrade or Downgrade click on the "Change level" button.
When you land on the next screen use the "Switch to this level"
links to select your best plan.