Open WhatsApp
Go to Settings
Tap Chats
Tap Chat backup
Tap Back up to Google Drive or iCloud settings if needed
Delete the backup from Google Drive on Android
Open Google Drive
Tap Menu
Tap Backups
Find WhatsApp backup
Tap More
Tap Delete backup
Delete the backup from iPhone iCloud if needed
Open iPhone Settings
Tap your Apple ID
Tap iCloud
Tap Manage Account Storage
Tap WhatsApp Messenger
Tap Delete Data
Turn off WhatsApp backup in WhatsApp if you do not want future backups
Open WhatsApp
Go to Settings
Tap Chats
Tap Chat backup
Tap Auto Backup
Select Off