At the bottom of the Visible app, there are three tabs: "Home," "Shop," and "Account." Under "Account," there is "Plan." Right there, there should be an option for you to change to a different plan.
With regard to contacting Visible Care, I tried FB Messenger and got a reply 24 hours later.
Operative word SHOULD. I’ve never had a plan option under account or shop. Under account all I have is the normal overview, nothing I can change except payment method; under shop—all I see are phones. For the brief time I had an option it was on the first screen after signing in and all I got trying it (multiple times) was the message saying they were unable to process my order & to contact customer care for resolution.