To do this you must be logged in as the Account Owner, and then click on Account in the upper right of your store admin.

Note: Your account assets must match the plan you are switching to, example if you have 200 SKUs and the plan you wish to switch to only allows 100 you must remove enough SKUs to qualify for that plan.

Overview of plan selection screen

At the time you upgrade or downgrade, an invoice is generated automatically to close the current billing cycle. That invoice is pro-rated: it will likely amount to less than what you have paid before for a full month of subscription. A new billing cycle begins on the new plan with a new subscription fee.