How can I do to install this IOS 15.8 on iPhone 6splus

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8

• If you have an iPhone 8 or X, you can update to 16.7.2

• If you have an iPhone SE (2nd generation or later), or XR through 15, you can update to 17.1.1

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support