why can't install ios 15.7.5 on my iphone 6s?

What happens when you try?

Make sure it's a 6S and not a 6 or 6plus

Can you post a screenshot of the whole screen, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• You have an active cell signal (You need both WiFi and cellular service to perform the upgrade).

• Are not using a VPN connection.

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 10 gigs of storage available.

Click on this link on what to do if you can't update your phone -

If your iPhone or iPad won't update – Apple Support (UK) If your iPhone or iPad won't update – Apple Support (UK)

Are you absolutely positive it is a 6s and not a 6 or 6Plus? To identify your iPhone model refer to the information in this article --> Identify your iPhone model - Apple Support

Somebody yesterday had this issue and it turned out they misidentified their device.

There are different ways to ensure the highest iOS that your device will support is installed on your device.

This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

You can also use Mac and PC computers to update system software:

Update iOS on iPhone - Apple Support

or:

Update iPadOS - Apple Support

When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date (and can't go any higher) and won't install anything. Apple indicates ( Unable to update my iPhone 6 Plus from 12… - Apple Community ) I need to point out the obvious and state that if you want to run a higher system version than your current device will run, that you need to buy a newer device.

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

Another, less common reason for not being able to update is if you were using a beta version of software. Refer to How to uninstall iOS or iPadOS beta software - Apple Support

If you're sure it is an iPhone 6s model (not a 6), from your product list (if updated) it looks like the iPhone is still running the prehistoric iOS 12 version (very odd).

If this is the case, you may find many troubles trying to update it to iOS 15 wirelessly (via wifi).

In such a situation, I suggest that you update it via computer (if you don't have a computer borrow one from your friends/neighbors, or ask for help at an Apple Store or Authorized Service Provider), then learn how to update your device manually if you're using a Mac with macOS Catalina or later, or if you're using a Mac with macOS Mojave or earlier or a Windows PC.

If you still get no help, contact Apple Support directly, using this link:--> Get Support

Regards

Giulio

I am trying to touch in about phone and touch in software verision but not shown any update

Hello~ What model iPhone are you trying to update?

~Katana-San~