High Sierra is now available in the App store for most Macs from 2009 onwards, if it’s not available to you in the App Store it is likely not an available update for your Mac, you can check manually the models supported here.
Most Mac users cannot wait to install and get running with the most elevated macOS so far. Follow these 6 steps for a Happy High Sierra!
Make a backup with Time Machine, don’t rely on an out of date backup make a fresh backup right before you start the High Sierra installer. Download the macOS High Sierra Installer from the App Store which is in > Applications > App Store.
You can run the installer after downloading it from the App Store or perform a clean install which will completely replace the version of macOS you are currently running , to do this you need to create a USB install drive.
MacWorld has written an excellent article on how to create a USB installer for
High Sierra: https://www.macworld.com/article/3204672/macs/how-to-create-a-bootable-macos-high-sierra-installer-drive.html
You may want to do this before running the installer! Check the compatibility of your Apps! If you rely on a certain version of Photoshop or any App for work you need to make sure it will be compatible with High Sierra or you will be wasting time installing and then uninstalling High Sierra! Apple’s own Pro apps are listed here for compatibility
Run First Aid! This seems dramatic but a corrupted file or permission on your chosen install Hard Drive could cause the installer not to complete, Simply open Disk Utility from Applications > Utilities , choose your install Hard Drive from the list on the left then First Aid from the buttons at the top and click run.
Update your Apps first! If you haven’t got all your Apps up-to date or you are not sure, open the App Store and click updates from the top menu (furthest to the right) then choose update all from the top of the updates tab.
Following these steps will make the upgrade process as smooth as possible, and the backup you create will potentially save your bacon when things go wrong. Enjoy using the most Advanced OS from Apple so far.