You can manage your subscription plan in your Apple ID settings, however, according to their policies, it can only be changed to a new one when your current subscription is up. So if you’d like to upgrade, you’ll first need to cancel your current subscription and then purchase a new one. Once you cancel, your current subscription will end once your current billing cycle closes. Then (or sometime before the expiration, if you wish to continue your current workout plan), you can switch to another subscription plan.
Please note, that to avoid being charged for the renewal, you will need to cancel your current plan at least 24 hours before the next billing period.
For more details, please contact Apple Support directly, as they manage all transactions and subscription plans.