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28,662 questions
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The 'remove' feature will only remove tools that are out of support. NET SDK 8.0.400 is still supported and therefore won't be removed. Not sure why you're getting an error installing the newer SDK just because the older one is installed. You can have any # of SDKs installed (across versions) without any issues.
The easiest thing to do is go to Installed Apps in Windows and uninstall the SDK from there. However note that if you don't have a newer SDK installed then it'll break stuff that relies on it (e.g. Visual Studio) although apps that use the runtime should be fine.
Alternatively you can remove the SDK by going to a developer command prompt as an admin. Run dotnet --list-sdks to see what is available. Then download and use the DotNet uninstall tool from MS as discussed here.
But I would recommend you get the newer SDK installed first. I suspect if you're getting an install error then it won't be fixed by removing the older SDK.