I want to upgrade my iOS to 15.7

If you have an actual iPhone 6s, as suggested by your tag line, then go Settings ➜ General ➜ Software Update

click here ➜ Update your iPhone or iPad - Apple Support

If you have an iPhone 6 (no s), then it is not capable of iOS 13 or later. It lacks that hardware, RAM and CPU for anything newer. The most current iOS version it can run, is iOS 12.5.7 at this time.

For sure check your iPhone model & iOS version -> Go to: Settings -> General -> About.

If your iPhone is an iPhone 6S Plus you can update the iPhone to iOS 15.7.4, see -> iPhone models compatible with iOS 15.5 -> iPhone models compatible with iOS 15.5 – Apple Support (UK)

You need about 6 GB device storage and good Service and/or Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

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, iPad or iPod touch won’t update

If your iPhone, iPad, or iPod touch 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