Unable to update my iPhone 6 to iOS 15

Unable to update my iPhone 6 to iOS 15

Correct. That would be the case since the iPhone 6 can only be updated as far as iOS 12.5.7.

To update to iOS 15, you would need an iPhone 7 or higher model.

How to Update iPhone on Computer - Lifewire

https://www.lifewire.com/update-iphone-on-computer-5206080

(apple support also has this, in several articles)

..Also note that iOS 15.8 can be used from iPhone 6s through 7Plus..

Maybe yours might need to use a USB lightning cable & PC/Mac?

Connect iPhone and your computer with a cable - Apple Support

Otherwise, try Software Update; and enter passcode, to update.

Works fine, with my iPhone SE (1st edition) model as indicated.

Anyway, if set up correctly in advance, it should auto-update

over-night; or if you need to monitor progress, as you wait.

[A screenshot from "Guilio's iOS Versions" upload, failed?.]

[Maybe the 're-titled by moderator' creates a different question here?]

You need about 8 GB device storage and good Service and/or Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

Go to Settings -> General -> Software Update -> Download and Install.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone or iPad won't update

If your iPhone or iPad won't update - Apple Support 

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

or see: