I cannot update WhatsApp, my iPhone requires an account password

Check your purchases history to see if the app was downloaded using your current Apple ID.

Purchase History Review.   

If it wasn't downloaded using your current ID, delete the app. Then sign out of the App Store/iTunes, restart the device, and then sign back in. Try downloading the app using your Apple ID.

Click on this link to read what to do "If you forgot your Apple ID password" --> https://support.apple.com/HT201487

or go directly to this link --> http://iforgot.apple.com/

Also read about "What to do after [or before] you change your Apple ID email address or password" - https://support.apple.com/HT204071

If you still need help:

Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." --> https://support.apple.com/HT204169