As soon as you have pressed the Power button, press and hold down the Shift key on your keyboard.

If your Mac isn’t already shut down, shut it down, and wait at least 10 seconds before pressing the Power Button to start it up restarting is not the best way to get this to work properly, although on many occasions you may get away with it.

We often don’t do that properly, and it’s worth following Apple’s instructions very carefully: In most cases the best thing to try next is to start up in Safe mode, which is explained in full here for Catalina, and here for older macOS. If you don’t try it now, don’t forget later on. In some cases, problems vanish after resetting the SMC and NVRAM, a quick and simple procedure which is worth trying sooner rather than later. If macOS does really seem messed up, shut down or leave your Mac until you have counted to a hundred, had a strong cup of your favourite beverage, and have worked out what to do next. If you were just about to go out, for example, don’t rush at anything in a panic. Although mis-updates used to be quite common in the past, they do still occur. Updating macOS is one of the most complex things that happens to your Mac, even when it’s ‘just’ a Security Update. The first and most important action is not to rush at trying to fix it, or give up in disgust and start reverting to the last version you were running.

Instead of getting the latest, fixed and sweet-running version, Software Update dumps its ghost on you, and macOS is staggering along looking like it’s half dead. It happens: just when you think it’s safe to update macOS (even just a Security Update), it turns round and bites you on the bum.

Os x version 10.13