After over an hour, I cannot determine where/how to switch from the subscription modal to the lifetime purchase. Consider updating your user interface/guidance if you want a user to manage their subscription, a critical/basic need of any application.
Agenda will present the options for lifetime if your license expires. If you have a valid license, it won’t show any options.
The simplest way to do it is to cancel your subscription in the App Store. Don’t ask for a refund, but just let it stop at the end of the year. At that point, Agenda will show all the purchase options you have.
If you don’t want to wait, first cancel the subscription, and then, in the Mac app, hold down the Option key while you choose the menu Window > Premium. You should then see all purchase options shown.