It looks like Mixer keeps some garbage after being updated. Since each update basically re-installs it from scratch.
How do I get rid of the excessive “Mixers” without damaging the installed software?
Hey there, I’m happy to help with this! Before we begin, I recommend duplicating your Megascans Library folder to a different location on your hard drive to ensure that no downloaded assets are lost during this process.
First, navigate to the folder where Quixel Mixer was installed and select the Uninstall.exe
There should only be one version of Mixer in each version folder. For example, the path “E:/Program Files/QuixelMixer-2021.1.2/” should have Mixer version 2021.1.2 only.
When uninstalling the application, you will receive a prompt asking you which of the components you want to uninstall.
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Entire application
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Individual components
What you will want to select is the “Individual components” option.
Once done, you’ll be shown a prompt displaying the components that are present in your system.
You can select any of those components and proceed. For example, you keep the sample mixes and the smart material components but select the application only. This will uninstall the application and the related installation files but will not touch any of your assets or project files.
If you run into any issues, please let me know. 
Hello!
I only have one version of Mixer installed on my system, I double-checked. And it’s in the same folder as usual, without a version number:
When I launch “uninstall.exe” I only get a prompt to uninstall material packs:
Understood, thanks for letting me know. The Quixel Support team is handling your ticket from here, please let me know if you do not see our response!
I have the same problem. I have Quixel Mixer 2020 8.01 GB + 2021 @ 4.13GB in systems_installed apps and I ca not seem to find the actual folders.
I have complety unistalled Quixel mixer using the unistaller. The modify and Unistall options are greyed out in tge unistall options. Please help!
I had the same problem and I solved it this way: go to control panel, programs, uninstall or change program and delete the old versions manually from the list, an error message should appear saying that the software seems to have already been uninstalled and asking you if you want to remove it from the list.
in any case these versions have no effect on the disk space actually occupied