Can I choose or downgrade macOS on a refurbished Mac?

You can't pick the macOS before you buy. The seller installs it, and most install the newest version the Mac supports. Once the Mac is yours, you can install an older version, but never one older than the macOS the Mac shipped with. RefurbMe lists that floor as the Default macOS under Product configuration on every Mac product page. Plan on starting from an erased disk, so back up first.

Can I choose the macOS before I buy?

Not in practice. The refurbisher wipes the Mac and installs macOS before it ships, and the listing rarely says which version. Most install the newest version the hardware supports, as our page on what macOS refurbished Macs come with explains. If you need one particular version, for an app that won't run on newer releases for example, ask the seller before ordering and plan to install it yourself.

What you can choose is the model. A Mac can't run anything older than its default macOS, so if you need an older version, buy a model whose default macOS is that version or earlier.

What is the oldest macOS my Mac can run?

The one it came with. Apple's page on downloading and installing macOS says you can download installers for "compatible macOS versions no earlier than the version that came with your Mac". An installer that is too old for your Mac won't install. The App Store dims the Get button or says it isn't compatible, and Terminal answers "update not found".

The newest version has a limit too. macOS 27 Golden Gate runs only on Apple silicon Macs, and Intel Macs stop at macOS 26 Tahoe.

How do I install an older macOS?

Apple describes two routes. Both work on Apple silicon and Intel Macs, and both need an internet connection.

From an installer. Apple's download page links App Store installers for macOS Sequoia 15 back to High Sierra 10.13. For macOS Tahoe 26, use Terminal: softwareupdate --list-full-installers shows the versions available for your Mac, and softwareupdate --fetch-full-installer --full-installer-version followed by the version number downloads one. Then follow Apple's guide to creating a bootable installer on a USB drive (Apple says 32 GB is enough for any version), and start the Mac from it.

• Apple silicon: shut down, then press and hold the power button until the startup options appear. Select the installer and click Continue.
• Intel: turn the Mac on and hold the Option key until the startup disks appear. If a Mac with the T2 chip won't start from it, set Startup Security Utility to allow booting from external or removable media.

From macOS Recovery. Apple's page on reinstalling macOS says which version Recovery installs:

• Apple silicon: the current version of the most recently installed macOS. If you upgraded and then erased the disk with Disk Utility, you might get the version you had before upgrading.
• Intel: starting up with Shift-Option-Command-R might give you the macOS that came with your Mac, or the closest version still available.

What does going back erase?

Expect to lose everything on the Mac. Apple doesn't describe installing an earlier version over a newer one with your files kept. On Apple silicon, its route back through Recovery starts with erasing the disk in Disk Utility. For the Intel route, Apple doesn't say whether your files survive. Plan it as a clean install: back up, erase, install the older version, then bring your files back.

Erase All Content and Settings won't take you back. Apple's erase page says it removes your data "while maintaining the operating system currently installed".

How do I back up first?

Use Time Machine. Connect an external drive, open System Settings, then General, then Time Machine, and click Add Backup Disk. Apple's Time Machine guide has the full steps.

Apple's page on restoring from a backup doesn't say whether a backup made on a newer macOS restores onto an older one. So also copy the files you can't lose to a separate drive or to cloud storage.

On a Mac that just arrived, there's little to lose. If the reason you want an older version is an app that won't run, test it in the first days, while the seller's return window is still open.

Can I update again later?

Yes. Updating is free through System Settings, General, Software Update, up to the newest version the Mac supports. So go back only when an app or device you rely on needs the older version. To see how long each model keeps getting new versions, read how long refurbished Macs receive software updates.