Does installing new macOS delete everything?

You can restore your Mac to factory settings by erasing your Mac, then using macOS Recovery, the built-in recovery system on your Mac, to reinstall macOS. Important: Erasing the volume removes all the information from it.

Does updating Mac OS Sierra delete everything?

No. Generally speaking, upgrading to a subsequent major release of macOS doesn’t erase/touch user data. Pre-installed apps and configurations too survive the upgrade. Upgrading macOS is a common practise and carried out by lot of users every year when a new major version is released.