I can’t download WhatsApp on my iPhone 6, and the version iOS 11.4 and I cannot find where to update it to iOS 12.5

You can update iPhone 6 to iOS 12.5.5., see the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

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

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

You need about 4 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.