If you've purchased the upgrade already and the app doesn't recognize this you can restore your purchase from the More tab and then Account, tap the Restore Purchase option.

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.
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If you need additional help please open a chat with me.