can my 2015 mac get software to update it

A late 2015 iMac can be upgraded to the latest version of Monterey macOS 12 or earlier but no later than Monterey. See: How to download macOS - Apple Support

Be aware that if you upgrade to Catalina 10.15 or later you will also need to update any non-Apple 32-bit apps. See: https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

Back up your data, check the application compatibility table, click here, and then on the download link for a more-recent OS. The maximum one for a late 2015 iMac is Monterey.

Catalina and above won't run 32-bit software; if you choose one of them, use Go64 as well.

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To be clear, software update is a revision of the macOS currently running on the Mac; update Catalina 10.15.1 to Catalina 10.15.7, for example.

To update your current macOS, click the Apple  icon > About This Mac and then click Software Updates.... If there are any updates available for your Mac, they will be listed and you can apply them when you like.  Software updates may be set to occur automatically.  If no updates are presented, then it may be assumed that none are available for your current OS.

A software upgrade replaces the current macOS with a newer, differently named version; upgrade from 10.13 High Sierra to 10.14 Mojave, for example, or from 10.15 Catalina to macOS 11 Big Sur. Upgrades do not happen automatically, They must be initiated by a user with admin privileges.

Use Software Update to update or upgrade macOS, including built-in apps like Safari. 

Please see >: Update macOS on Mac - Apple Support

❗️ Do NOT upgrade…

until you have made a Time Machine or other backup of your startup drive.

Learn How to Back up your Mac with Time Machine

How to Upgrade  macOS:

The macOS version you can upgrade to depends on the year your Mac was released.

Click the  (Apple) menu at the top of your screen and choose About This Mac.

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.13 High Sierra is compatible with the Macs listed here

➡️ macOS 10.14 Mojave is compatible with the Macs listed here

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here

➡️ macOS 12 Monterey is compatible with the Macs listed here

➡️ macOS 13 Ventura is compatible with the Macs listed here

❗️ – IMPORTANT – None of your 32-bit apps will work in macOS Catalina or newer. This includes MS Office apps and any other older software apps and drivers. These will require that you find 64-bit updates and/or replacement titles.

32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

Use the free utility Go64 to check the compatibility of your apps:

https://www.stclairsoft.com/Go64/

✅ - IF your Mac is compatible, then download the macOS upgrade installer app:

Click > High Sierra,  Mojave,  Catalina,  Big Sur,  Monterey  or  Ventura.

Once downloaded, double-click the “Install macOS…” app in the Applications folder to begin the upgrade.

Please see > How to download macOS.