Upgrading/downgrading an item in your subscription means that you have the option to switch between variations of the same product. This option is only available to specific items, and will appear to you in the form of a button.
Clicking this will bring you back to the product page, where you can select another variation of the product to replace your subscription.
To confirm your selection, click to proceed with the button "Sign Up Now".
Proceed to the checkout to confirm your selection. You will not be charged, but your subscription will be amended.
You will be brought to the confirmation page, where you can click to view your amended subscription and future billing amount.
Clicking to view the subscription will show the amended subscription and new order total!