Upgrading a macOS depends upon the model Mac, specifications, and year. To get more information about your computer, choose: (Apple menu) > 'About This Mac' in the upper left corner of any window, then "More Info..." or "System Report". There's more about this in System Information User Guide for Mac - Apple Support
Use the computer information, including the model year, and the links below to determine what is possible with your computer.
Click on this link to read how to get different versions of macOS --> How to download and install macOS - Apple Support
- The article also has links to system requirements for each OS version which you should check against your exact computer model. This web site also has a reliable presentation of model and OS version compatibility: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility
Realize that your computer may simply be too old to run the version currently available in the app store. Sometimes an older version is available, but if it is not then you only option if you wish to use that app will be to buy a newer computer.