imac OS X 10.9.5 late 2013 Need to upgrade OS! please help

How to get Older versions of MacOS: https://support.apple.com/en-us/HT211683

I'd suggest either Sierra, High Sierra, or Mojave.

El Capitan is old enough your are likely to still have compatibility issues with modern software, while Catalina is so new you are very likely to loose support for much of your existing software.

Also if you are using a 21.5" model with a 1TB HDD (not fusion or SSD) then I would further suggest you purchase an external USB 3.0 SSD to install MacOS onto. The 5400rpm HDDs have been causing performance problems for the newer MacOS versions.

Back up your data, check your applications for compatibility, and click on El Capitan, High Sierra, Mojave, or Catalina.

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