If there is an update for the App download it.
you can delete the app by holding down on the icon and clicking (X) and then downloading it again
or you can reset the phone to factory settings and see if it works
Reset your iOS device to factory settings - Apple Support
Resetting restores the device to factory settings, erasing all music, video, contacts, photos, and calendar information.
past that you would need to speak with the developers of the product.
WhatsApp is responsible for their product working in iOS. Apple is not in any way responsible for making iOS work for WhatsApp. The developers had iOS11 months ago to either work it our or not.