If you are on Ventura or a new version of macOS, please see: How to Update Apple Computers Post Ventura Update
1- Click on the apple logo on the top left-hand corner of the screen.
2- Click on the About This Mac option.
3- Click on the Software Update... button.
What do you want to do next?
Upgrade to the next version of macOS? Go to step 4
Update your current version of macOS? Go to step 6
Upgrade to the next version of macOS?
4- If you need/ want to upgrade to the next version of macOS click the upgrade now.
5- You will then be prompted to restart your mac once the download is complete. Once your computer boots back up you will need to set up the new version of the OS. It will ask you a few question, answer them and then you will be back in the desktop.
You are now done.
Update your current version of macOS?
6- If you are updating the current version of macOS click on the More info... option
7- Click on the Install Now button.
Depending on the update, you may or may not be prompted to restart your mac.