Updating the operating system on my 2015 macbook pro

Software Update may only show updates to the current main system version you are running, not upgrades to a higher system version such as a jump from El Capitan to Sierra.

Click on this link to read how to get other versions of macOS - https://support.apple.com/HT211683

- This article also has links to system requirements for each OS version which you should check against your exact computer model. Also make sure you have more RAM than what is noted as "minimum" for any particular system or your computer may run slowly.

- You may find you need to use Safari to get any links using the Mac App Store application to open a functioning page.

Back up your data in case something goes wrong --> Back up your Mac with Time Machine - https://support.apple.com/HT201250 or use a cloning tool.

Check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer.

System Preferences> Software Update did not come into being until macOS Mojave 10.14.

In Yosemite software updates were handled through the App Store Updates page.

The App Store Updates page only informs you of updates available for the OS you are running.

Installing a newer OS is called an upgrade.

Which OS you can upgrade to will be governed by which model and year your mac is.

Your 2015 mac can upgrade to any of the available OS's including the latest macOS Big Sur.

If you want to upgrade to Mojave, Catalina or Big Sur I would suggest upgrading to macOS High Sierra

first.

For older Os's,

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to Check Compatibility to see if your mac qualifies for the OS upgrade you want.

Then go to the Download OS section and click on the OS you want to install.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you want to or are able to install Big Sur, open Safari and click on the link below.

How to upgrade to macOS Big Sur - Apple Support

If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see

if they are offering 64bit updates.