I need to change my passcode on iPhone but the “face Id and passcode” option is not available. What do I do?

iPhone 6s does not have Face ID capability.

Try....Settings > Touch ID & Passcode.

OK, your profile said 6s, so we took that at face value. Is this your phone, or is it a phone that your employer provided to you?

On an iPhone 12, I see Settings > Face ID & Passcode.

You may need to contact Apple Support directly for more info:

Support