My update is not showing

If you see your software is up to date your iPhone has running the latest version it can run

For the iPhone 6 this is iOS 12.5.7

For the iPhone 6S this is iOS 15.8

If you're still running one of these software versions you cannot update further

If you're not running one of these software versions you might want to update via a pc or Mac. Or is your iPhone company managed? This could also cause the software update function to disappaear

Update your iPhone using a Mac with macOS Catalina or higher

On a Mac with macOS Catalina or higher, you will use the finder in order to upgrade your iPhone. Make sure to Back up your device before starting the upgrade process.

On a Mac with macOS Catalina or later, you will follow the following steps:

1. Open a finder window
2. Connect your device to your Mac via a lighting to usb A or a lightning to usb c cable On the iPhone 15 series, use an usb c to usb c cable.
3. Select your device in the left sidebar of the finder window
4. click general, then click check for update
5. Click download and update.
6. If asked, enter your iPhone passcode

Update your iPhone via a Mac running macOS Mojave or earlier, or on a Windows PC

On macOS Mojave or earlier, or on a windows machine, we won't use the Finder to upgrade your iOS

Instead, we will use iTunes. Make sure to Back up your device before doing any update or upgrade to your phone.

1. Open iTunes
2. connect your device to the computer with an lightning to usb a or a lightning to usb c cable. On the iPhone 15 series, use an usb c to usb c cable.
3. Click device button in the iTunes toolbar
4. In the summary tab, click check for update
5. Click download and update, follow The on screen instructions
6. If asked, enter your iPhone passcode.

See this user tip I wrote about updating for further reference

Updating/upgrading your iPhone - Apple Community

If you see a message that your iPhone's software is up to date, this would indicate that your iPhone 6s already has the most recent software available for your iPhone.

What version of iOS is currently installed on your phone?

The iPhone 6s cannot run iOS 16 or iOS 17....if that might be what you are looking for....so you will never see an option to update your iPhone with those versions.