You read that right! This is a complete guide on how to upgrade from OS X Mountain Lion 10.8.5 to newer macOS versions.

First, you need to identify your Mac model. To do this, click the Apple icon → About This Mac → More Info → System Report.
Depending on your Mac identifier, you’ll have one of two upgrade paths:

Upgrade Options

1. Upgrade directly to OS X El Capitan (10.11)

2. Or upgrade first to OS X Yosemite (10.10) if your Mac does not support El Capitan directly.

Important Note

Sometimes Mountain Lion does not allow you to use Safari, and you may not even be able to sign in with your Apple ID. In that case, you’ll need to download an older version of Google Chrome (download link provided here).

Once Chrome is installed, download the installers for El Capitan 10.11 or Yosemite 10.10. Try the alternate source linked here.

If You Get the El Capitan Verification Error

If you try to install El Capitan and see the error:

“This copy of the Install OS X El Capitan application can’t be verified. It may have been corrupted or tampered with during downloading.”

or

"You will need 10.15..." 

This usually means one of two things:

1. Your system date is incorrect

Open Terminal (click the magnifying glass on the top-right and type Terminal).
Type:

date

If the date is wrong, correct it using:

date MMDDHHmmYY

Where:

• MM = month (01–12)

• DD = day (01–31)

• HH = hour (00–23)

• mm = minutes (00–59)

• YY = last two digits of the year (e.g., 16, 17, 23…)

And try again to install El Capitan.

2. Your version of Mountain Lion does not support the installer

If the date is already correct or if you fixed it and you still can't install El Capitan, it may mean your Mountain Lion build cannot verify the certificate used in the newer installer. In that case, follow the recommended upgrade path and install Yosemite first.

Good news is, now that you have either El Capitan or Yosemite, you can now use OpenCore Legacy Patcher (Download it here in case you can't find it) but usually you may find it here, since the OpenCore is only available from Yosemite upper. Now, follow the steps:

• Make sure you have a USB that has, at least, 20GB of space.
• Open OpenCore Legacy Patcher.
• Choose to create macOS installer.
• Select to Download macOS installer and choose Big Sur.
• Select the installer you just downloaded.
• Select the USB drive.
• When it finishes, go back to OpenCore Legacy Patcher, and select Build and Install OpenCore.
• Select the USB drive.
• When it reboots, select the Efi Boot from the USB drive, and then install the OS.
• When it reloads, go back again to OpenCore Legacy patcher, Download macOS installer, and now you can update to the desired OS.
• Select the installer you just downloaded.
• Select the USB drive.
• When it finishes, go back to OpenCore Legacy Patcher, and select Build and Install OpenCore.
• Select the USB drive.
• When it reboots, select the Efi Boot from the USB drive, and then install the OS.
• When it reloads, go back again to OpenCore Legacy patcher, and select Build and Install OpenCore.
• Select your SSD and not the USB drive this time.
• After the reboot, go to OpenCore Legacy Patcher one last time, and select Post-Install Root Patch.
• When the final reboot ocurrs, select the terminal and type this:
• sudo softwareupdate --schedule off
• sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate AutomaticDownload -bool false
• sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate AutomaticCheckEnabled -bool false
• sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate AutomaticDownload -bool false
• sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate AutomaticallyInstallMacOSUpdates -bool false
• sudo defaults write /Library/Preferences/com.apple.commerce AutoUpdate -bool false
• sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate ConfigDataInstall -bool false
• sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate CriticalUpdateInstall -bool false
• This is to ensure your computer no longer updates to newest versions, so the system doesn't get corrupted.