For Ubuntu:

• First list the dotnet packages installed:

sudo apt --installed list | grep "dotnet"

• You'll have a result like this:

dotnet-apphost-pack-3.0    dotnet-hostfxr-3.1         dotnet-runtime-deps-5.0
dotnet-apphost-pack-3.1    dotnet-hostfxr-5.0         dotnet-sdk-5.0
dotnet-apphost-pack-5.0    dotnet-runtime-5.0         dotnet-targeting-pack-3.0
dotnet-host                dotnet-runtime-deps-2.2    dotnet-targeting-pack-3.1
dotnet-hostfxr-2.2         dotnet-runtime-deps-3.0    dotnet-targeting-pack-5.0
dotnet-hostfxr-3.0         dotnet-runtime-deps-3.1

• To uninstall the packages of linked to version 3 for example just type:

$ sudo apt-get remove --purge dotnet-*3*

Credit

https://stackoverflow.com/a/43232120/1872200