Update iPhone 6 iOS 12.5.7

Hello,

Your device will update to the highest version currently available that it will support. and for an iPhone 6 that is iOS 12.5.7. Here are two different ways to ensure the highest iOS that your device will support is installed on your device.

For instructions about updating your iPhone, iPad, or iPod touch wirelessly, click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Indeed, but since you have an iPhone which won't update beyond iOS 12 you will not be able to run that app until you buy a newer iPhone.

How to update iOS 12.5.7 to iOS higher

How to upgrade iOS