I want to update my iPhone 6

Only iOS 12.5.7 is currently available for an iPhone 6

Here are two different ways to ensure the highest iOS that your device will support is installed on your device.

Update 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

Your device will update to the highest version currently available that it will support.