It says I cannot download iOS 15.7 in iPhone 6

Depending on the model iPhone you have, the age of that model

would limit an update ~ iPhone 6 cannot use newer iOS software.

iOS 12.5.6 is latest system for iPhone 5s, 6, 6Plus; these cannot use

newer iOS 15.7 due to build date. Newer iOS supersedes old iPhone.

See the Apple Support article:

• Identify your iPhone model - Apple Support

//support.apple.com/en-us/HT201296

• About the security content of iOS 12.5.6 - Apple Support

//support.apple.com/en-us/HT213428

• iPhone models compatible with iOS 16 - Apple Support

//support.apple.com/guide/iphone/supported-models-iphe3fa5df43/ios

.You'd need to buy newer iPhone, to use later compatible iOS system. [Your iPhone6 can't run iOS15.7..]

PS: iForgot to include this link ~ before window of time passed:

• Update your iPhone or iPad - Apple Support

//support.apple.com/en-us/HT204204

This includes method instructions, to update by Wire; and how

to differentiate which iOS and computer OS are involved there.

[.Many saved links, make One harder to find, from bookmark folder.

.also: thanks to Guilio for use of his iOS iPhone chart update here.]