Make Sure You Know Your Apple ID and Passwords
When you install the macOS High Sierra update you will need to enter your Apple ID and password to complete the setup. This is a security step that you will need to take if you want to use iCloud services and iMessages on your Mac.
If you don't already know your Apple ID and password we recommend finding it and or updating it before you install macOS High Sierra. You can go to iCloud.com and login with your information. If it works, you are good to go, but if it doesn't you'll need to follow the instructions there to reset it.
You may also need to enter your username and password for some apps and maybe even your mail accounts. If you don't have these saved, we recommend checking on your five most used accounts before installing the upgrade.