There are three possibilities for upgrades/downgrades:
Change the plan by editing the existing subscription.
Set up add-on in an existing subscription.
Increase/decrease the price of an existing subscription.
In the sidebar, click on Customers > Customers.
In the Customer Detail page, click on the Subscription to be changed.
In the Subscription Detail page, click on Edit.
Change the part of the subscription:
If there are several plans, change the plan.
Change the pricing.
Click on Add Add-On to add an add-on.
Change options:
On next renewal: according to the setup of the current plan.
Immediate: decide on the compensation of the amount the customers already paid:
No compensation
Compensate with a refund: Prorated (the rest of the period) or full refund
In the Billing options decide what happens with the remaining period.
Click on Submit to confirm.
All functionality is also available through the API if that is relevant: