Why doesn't Adobe let me update my Creative Cloud Plan to 1TB

You must cancel your old plan before you can order a new one.  Pick up the phone and call Adobe Support for assistance.

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How do I Cancel my Subscription?
https://helpx.adobe.com/manage-account/using/cancel-subscription.html
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Subscription & Cancellation Terms:
https://www.adobe.com/legal/subscription-terms.html

CONTACT ADOBE SUPPORT:
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Online Chat: https://helpx.adobe.com/contact.html?rghtup=autoOpen
Phone M-F, during normal business hours: https://helpx.adobe.com/contact/phone.html

Sorry, nobody here can help with an account or payment problem
This is a public forum, you need the link to Adobe support, not other users
Be sure to remain signed in with your Adobe ID before accessing the link
Do be aware that Adobe's staffing is impacted by the virus, so wait times are long
-make sure that your browser does NOT block ads, scripts, or pop-ups
-you must also allow 'cookies' in your web browser for the link to work
This link will go directly to the chat option
Type "Agent" in the chat box to bypass the chatbot and be connected to a person
-Meaning type in the word Agent without the quote " " marks
https://helpx.adobe.com/contact.html?rghtup=autoOpen
- or https://helpx.adobe.com/contact/phone.html
.
Also: beware of answering anyone who sends you a private message
-click for more information about scammers https://tinyurl.com/10791730