Does updating mac os delete everything
Immediately hold down the Option key until the boot menu appears. When you arrive at the mac OS Utilities screen select Disk Utility then click Continue Select Macintosh HD or the name of your startup drive then click Erase. This is important because High Sierra modifies the drive making it impossible to even restore from a Time Machine backup. Select Restore from Time Machine Backup then follow the instructions for restoring your backup.After your Time Machine Backup, restart your Mac and you should be back in your old version.If your Mac is running mac OS 10.12 (Sierra), 10.11 (El Capitan) or earlier, make sure you perform a full time machine backup.High Sierra will convert the file system your Mac currently uses called HFS to the new APFS.Unlike i OS though, mac OS is a more complex creature; due partly to its need to support third-party applications that are not as curated and carefully managed as those found in the app store on i OS.
Having a bootable copy of mac OS Sierra on hand can help.
In addition to the new i OS version 11, Apple also released its latest revision of its desktop operating system, mac OS High Sierra.