Log into WhatsApp with the same Phone number that was used to backup. If it prompts you to update chat history, then you use it or else check the Phone Number. WhatsApp chat history can be deleted via WhatsApp only.
Even if you erase your iPhone the chat history remains on iCloud Drive and is accessible only through WhatsApp.
WhatsApp Backup is taken on iCloud and while restoring, it verifies your Phone number.
- The phone number used for backing up and restoring must be the same. You can't restore the chat history from another WhatsApp account.
- Because backups are tied to individual phone numbers, it is possible to have backups for multiple WhatsApp accounts stored in the same iCloud account.
Contact the app developer here --> App Support or WhatsApp FAQ Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp