To change your Celtx plan, log into your studio account at celtx.com and select 'Management' then 'Plan' from the left hand side of your screen. Here, you can choose to upgrade or downgrade your level of service.
Plan upgrades and additional features are available in the "Improve Your Creative Experience" section.
You can upgrade to a plan that suits your needs regarding studio collaborators, additional tools, or a full plan change. Upgrading your service will calculate a pro-rated purchase difference.
Also on this page, plan downgrades, add-on removal, as well as many upgrade options are available in one spot. Scroll down to the "Review Your Billing" section near the bottom of the page and select "Change Plan":
Here you'll see quick options to select the plan that fits you best right now.