You can choose a new subscription plan from Admin > Start Subscription.
Please make sure that you use the same PayPal email you used to subscribe initially so that PayPal gives you an option to modify the existing subscription rather than creating a new one. When you modify your existing subscription, your account limits get upgraded instantly
but the new amount will be charged on your next payment due date.
In case you want to use a new PayPal email, please cancel the existing subscription using Admin > Cancel Subscription first using your old PayPal email and then start a new subscription from Admin > Start Subscription using your new PayPal email.