There are two things you need to do.
1. Contact 1tap support and tell us about it.
Email us at firstname.lastname@example.org
Chat with us on our Facebook page:@1tapreceipts.
Contact us directly from the app, just tap the 'support' button in the settings menu.
2. You will need to get a refund for the Prime 50 subscription.
The App store/ Google Play Store will refund you the money for your purchase. After the refund, you can purchase the Prime Unlimited subscription as per normal.