If you want to switch from the monthly to the yearly Structured Pro plan (or the other way around), you can easily do so by opening your settings ⚙️ and using the Manage Subscription button in your Structured Pro page.
Unfortunately, a direct switch from the monthly or yearly Structured Pro plan to the lifetime plan is not possible. In order to change your subscription to a lifetime plan, you first have to cancel your current subscription and wait for it to expire. After this, you can purchase the lifetime plan.