How do I update my iOS?

I see you are already running iOS 12 and your iPhone 6 Plus can only update as far as iOS 12.5.7 so you may already be running the highest version currently available for that model.

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

If your Apple mobile device won't update, read this support article --> If your iPhone or iPad won't update - Apple Support

Good Wi-Fi connection. Battery charged.

Settings > General > Software Update

The most recent update for the iPhone 6 Plus is iOS 12.5.7. iPhone 6 Plus cannot update to iOS 15.