I'm trying to restore my premium upgrade, but the app says "Restore failed".

The "restore failed" message usually signifies a temporary App Store glitch and the restore completes successfully when trying at a later time.

Unfortunately, the transactions are handled by the App Store and not directly by the developers, so we are not able to fully troubleshoot the issue. 

Apple has in place a guide for failed purchase restore at https://support.apple.com/en-us/HT204530.

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