Unable to install updates on my iPhone 6splus

Hey there,

See>>>Update your iPhone, iPad, or iPod touch - Apple Support

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if you have trouble with update See>>>If your iPhone, iPad, or iPod touch won't update - Apple Support

if you have iPhone 6plus not 6 plus S or less you can’t update it to newer version the last update for this device is iOS 12.5.5

The iPhone 6Splus is capable of running iOS 15 .

If you own a Mac you can try running the update from the mac., that helped with my mothers iPhone 6S

If you have a iPhone 6 without the S, you cannot run iOS 15