The main admin can change the plan of the subscription clicking on the profile image and then selecting Subscription. Click on Change plan and select the new plan, the billing cycle and the licenses (users).
Please note that changing a subscription plan can result in prorated charges. We use the start of the current day as the time from which we will calculate the proration, unless the start of the day is earlier than the start of the current billing period, in which case we use the start of the next day.