What is the latest update for Macbook Air 2015? Click Reinstall macOS and wait for macOS Catalina to be installed on your machine. Your Mac should then open in Recovery mode. Select the bootable installer and click Enter. Restart your Mac while holding Option/Alt (depending on when it was made) to access Startup Manager.
How do I install the latest version of OSX on an old Macbook? … Download the Installer from Apple directly to the computer that you want to install the new OS X onto. This means if you have any version of OS X newer than 10.9 then you can upgrade it to the latest version for free. Since OS-X Mavericks (10.9) Apple have been releasing their OS X upgrades for free.
Psssssst : How to backup my iphone to my macbook pro?