How to update iPhone 6 software

To update to the most recent, compatible version of iOS 12.5.7, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

WhatsApp is compatible with iOS 12 or later, so you can run it on your iPhone.

The tags indicate you have a iPhone 6 with iOS 11 installed. You can update your iPhone to iOS 12.5.7 by going to Settings -> General -> Software Update and installing the particular update.

Make sure your iPhone has

• charged to at least 50%
• preferably connected to the charger
• disabled any VPNs
• disabled Low Power Mode
• connected to a fast, reliable Wi-Fi

Then you can download WhatsApp on App Store.