Why will this new iOS update download on my phone
Why the new iOS update will not work on my phone
For 6s, Latest IOS version available is 13.5.1.
To update , Go to settings—- General —- Software Update.
your phone should be connected in wifi and battery charge should be above 50% to start the installation. Hope it helps. Thank you.
Check this link https://support.apple.com/en-in/ios/update