There's a couple steps that have to be taken before you can update to High Sierra. Here's what to do.

To begin, download the High Sierra update from the Mac App Store, but don't install it just yet.

First, you have to update Clover to version 4173 or higher (a High Sierra compatible build).

  1. To do this you have to start by Mounting your EFI partition so that you can back up your configuration and kext files.

  2. Once mounted, transfer your config.plist to a flash drive or some other safe storage media, as well as copy your kexts folder over. You will need these later.

  3. Install the latest version of Clover from SourceForge, make sure you install it to your mounted EFI partition.

  4. Once it's installed, copy your config.plist back over to its previous location, and in the kexts folder, ensure there is a 10.13 folder, and put your kexts there.

Secondly, you need to decide whether you want to use APFS or not.

  1. If you do want to use it, you will have to download APFS.efi and extract it to your drivers64UEFI folder on your EFI partition.
  2. If you do not want to use it, open the Terminal.app and run the following command: /Applications/"Install macOS High Sierra.app"/Contents/Resources/startosinstall --converttoapfs NO --agreetolicense, this will begin the installer without doing the APFS conversion. Your system will reboot, once that's done select the Install Mac OS boot option from Clover and you'll be on your way to installing High Sierra.

Once you decide whether to use APFS, you can install the update. However, if you are using a Nvidia GPU, you will have to install the web drivers for High Sierra. Keep in mind that System Integrity Protection must be enabled for the driver installation or you may face graphical issues.

That should be it for updating to High Sierra. Did it work for you? Let us know your experience in the comments section below.