Go to Settings => Upgrade, and tap the "Restore Purchases".
You can restore your upgrades for free when you change your devices or re-install the app, as long as you use the same Apple ID or Playstore ID. Restoring on Android is automatic, but please use the "Restore" button on iOS.
@Do I need to buy Taxnote Cloud again on different platform?
No, you don't need to buy Taxnote Cloud again when you switch from iOS to Android or from Android to iOS. You can use the same Taxnote Cloud account you make after subscribing to Taxnote Cloud to log into your account and sync your data.
@Do I need to buy Taxnote Plus again on different platform?
Yes, you need to buy Taxnote Plus on each platform when you switch from iOS to Android or From Android to iOS. You can restore your Taxnote Plus for free when you change your devices on the same platform like iOS only, but you need to purchase Taxnote Plus again when changing the platform.
*If you couldn't restore our purchases even though you are sure that you are logged in with the same apple id, please re-install Taxnote and try again.