How to revert to older version

I just updated to 2.0.2, and it doesn’t appear to be compatible with the Smart Slider 3 plugin. How do I revert back to an older version? (I had 1.8.1, I believe.) I know how to find and download the .zip file, but when I try to install it, I get an error because I have 2.0.2 installed already.

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