How long does a macOS update take?

Allow one to two hours for a major upgrade and around twenty minutes for a smaller security update. The Mac restarts several times and the screen can stay black for long stretches. That is part of the process.

Do I have to back up before updating?

Yes, do it. An update nearly always goes fine, but the one time it does not, a Time Machine copy on an external drive is the difference between a lost afternoon and lost photos. The first backup takes about an hour.

Should I install the very newest version on an older Mac?

If Apple offers the version for your model it will work, but on a Mac seven years old or more it can feel slower than what you have now. Sticking with security updates for your current version is often the calmer choice.

Update macOS safely, and what to do if it gets stuck

A macOS update takes one to two hours. During that time the Mac restarts several times, the screen can be black for long stretches, and the progress bar often sits still. That is normal and does not mean anything has stopped.

Do three things before you start and the rest is straightforward.

Try this first

Back up. Connect an external drive, open System Settings, General, Time Machine and add the drive as your backup disk. Let it finish. If you have no external drive, at least copy the Documents, Desktop and Pictures folders somewhere, or into iCloud.
Check your free space. Apple menu, System Settings, General, Storage. You need at least 25 GB free for a major upgrade. Empty the bin and clear out the Downloads folder if space is tight.
Put the Mac on the charger and leave the lid open. An update running on battery can be cut short halfway.
Start the update. System Settings, General, Software Update. Click Update Now for a smaller update, or Upgrade Now if it is a new version of macOS.
Type your Mac password in yourself when it is asked for. Never give it to anyone, including us.
Leave the Mac alone. Do not close the lid, do not press the power button and do not unplug the charger until you are back at the login screen.

If that did not help

If the update is sitting still:

Wait first. An hour with nothing visibly happening is still within normal for a major upgrade. Listen for the fan and feel whether the Mac is warm, which means work is going on.
If nothing has moved for over two hours, hold the power button until the Mac switches off. Wait ten seconds and start it again. The update usually picks up where it was.
If the Mac will not reach macOS, start it in recovery mode. On a Mac with Apple silicon, meaning M1 and later: switch off, then hold the power button until it says Loading startup options, and click Options. On an older Intel model: hold Command and R as soon as you start it.
Check the drive from there. Choose Disk Utility, select your drive in the list on the left and click First Aid. It takes a few minutes and deletes nothing.
Reinstall macOS if needed. Go back to the same menu and choose Reinstall macOS. It normally keeps your files and apps, but only do it when you have a backup.
If the update will not download, check your internet and try another network. Also check that the date and time are right under System Settings, General, Date and Time.

One step never to take alone

The same menu contains Erase Mac. Never run it without knowing the password for your Apple Account. Activation Lock asks for it when the Mac restarts afterwards, and without that password the machine cannot be used again, not by you and not by anyone else. That holds even when the Mac is your own and registered in your name.

If after three hours the Mac still will not reach macOS, if Disk Utility reports something it cannot repair, or if you have no backup, stop and ask a person first. Files can almost always be read out of a Mac that will not start, but it is easier before more attempts have been made.