UNABLE TO UPDATE IPHONE 6s plus

How is the range of the mobile network?

See this Apple article: If you see No Service or Searching on your iPhone or iPad

https://support.apple.com/en-us/HT201415

You need strong Wi-Fi network, check the Wi-Fi network.

See this Apple article: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

https://support.apple.com/en-us/HT204051

Turn Low Power Mode-> Off.

You need about 6 GB device storage.

Or try to update via a computer -> iTunes.