my iPhone 6s was not update pls help me update iOS version to 15.1

You can update your iPhone 6S to iOS 15.8.

You need about 8 GB device storage and good Service and/or Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

Go to Settings -> General -> Software Update -> Download and Install.

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 or iPad won't update

If your iPhone or iPad 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

or see:

Are you sure you have a 6S and not just a 6?

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.