If you've purchased the upgrade already and the app doesn't recognize this there are a couple ways to restore your purchase.
From the More tab and then Settings, at the bottom is a Restore Purchase button. Tap that and the app will restore any purchases you've made.
From the Calendar tap the Go Premium button. A little way down (you may have to scroll) is a Restore Purchase link. Tap that and the app will restore any purchases you've made.
If you originally purchased it on iPhone and now have an Android or the other way around the app will not be able to validate your purchase. Or if you've changed Apple IDs or Google Accounts from the one you purchased it on. In either of these cases please contact me in the chat from the more tab and I'll fix it.
If you need additional help please open a chat with me.