How do I fix this error?

Hello I started an update on my MacBook Air 2012, the update was pushed in and seemed like it was going until I received a message saying “ macOS could not be installed on your computer. An error occurred installing macOS. To use Apple Diagnostics to check your Mac hardware, shut down, press power button and immediately hold D key. . . .,” I tried to press the power button and the D key but nothing happens, the laptop just restarts again, it takes me to the login where I put in my password and then back to this error message. What should I do to fix this?