Whatsapp update not getting deleted I noticed my contacts list have been jumbled up meaning,
many contacts name are displayed with a common name.

So I erased all the phone data and factory reset.

Still the WhatsApp update icon is present and I am not able to delete it.

Why is this happening?
Whatsapp is installed on the /system partition on your device. Which means, your device manufacturer decided to include the app in the device firmware. A factory reset can only remove the apps which you installed yourself which are present on the /data partition. To remove system apps you need to need to root your device and delete the app from the /system/app/ directory.