How to update software on my iPhone?

The iPhone 6 can be updated to iOS 12.5.6.

Assuming you have a compatible device, 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. 

Go to Settings > General > About > Version. If it shows 12.5.6 that is as high as you can go. The 6 series phones do not have sufficient RAM to run iOS 13 or later and you cannot add it.