Microsoft Teams on my iMac

If your model is from 2012 or later, as shown in your profile, then it can be upgraded to Catalina: How to upgrade to macOS Catalina. However, if it is actually a 2008 model, then it can only be upgraded to El Capitan: How to upgrade to OS X El Capitan - Apple Support.

Before You Upgrade macOS

1. Fix any problems before upgrading.
2. Run First Aid in Disk Utility while booted from your Recovery HD.
3. Backup. Make a bootable clone using CarbonCopy Cloner or use TimeMachine.
4. Be sure your startup disk has 18-20GB of free space during the installation.
5. Upgrade third-party software for compatibility. See App Compatibility Table - RoaringApps. If you have mission critical software, then DO NOT upgrade until you know that software is compatible with the version of macOS you are about to install.
6. Disconnect all third-party peripherals.
7. Download the installer which will be in the Applications folder. Quit the installer after it opens, then make a copy of the installer and save it in the Downloads folder. The one in the Applications folder will be deleted after a successful installation.
8. The download is large, anywhere between 5 and 9GBs. It takes time to complete. Be patient.
9. Double-click on the installer in the Applications folder. Click on the start button to begin the installation.

The latest you can upgrade to is OS X 10.11.6; you can get it from here as long as you use the Safari browser to get it.