how to update

iPhone 6s Plus, iOS 13

If you are still on iPhone 6S Plus, you can update 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:

In principle you can use Facebook, Facebook requires iOS 13.4 or later. 

You can update your iPhone by selecting Settings > General > Software Update. If this does not work, please check the link below for other options.

Update your iPhone or iPad - Apple Support