Why can’t I update my iPhone 6s

The iPhone 6s can update to iOS 15, the iPhone 6 cannot update further then iOS 12.5.7

If you have indeed a 6s go to settings>general>software update and make sure to turn off low power mode and to connect your device to a wifi network.

If updating wirelessly is not working please update via a pc or Mac instead

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 this subject for further reference

https://discussions.apple.com/docs/DOC-250005397

If your device won't update, make sure you are following all the conditions for the method you are trying to use. If you are sure you are following directions correctly, try a different method.:

Update your iPhone or iPad - Apple Support - wirelessly*

Update iOS on iPhone - Apple Support - with a computer for an iPhone, or a related computer one for an iPad Update iPadOS - Apple Support

If your Apple mobile device won't update, read this support article for a list of possible reasons --> 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 need more space for an update read this support article --> If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

If your update feature on a device is greyed out or restricted, it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution. Read this document about using Screen Time on an iPhone, iPad, or iPod touch --> Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support

Tap Settings > Screen Time > Content & Privacy > scroll to Allow Changes.

* From ASC contributor Lawrence Finch at How can I upgrade my iPhone 6s iOS13 to i… - Apple Community here's a checklist of criteria that all should be met in order to update wirelessly:

• You need at least 10 GB of free storage for iOS 16 or 6 GB for iOS 15 or lower; more is better

• You must be connected to a Wi-Fi network (and not a personal hotspot)

• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update

• The phone must not be in Low Power Mode

• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles

• The phone cannot be in Airplane mode and must have a cellular data connection

• You cannot still have a beta version installed on the phone

• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it.