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$env:VariableName = "VariableValue"
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$envName = "MY_VARIABLE"
$envValue = "1234"
Set-Item -Path "env:$envName" -Value $envValue
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reload PATH only:
$env:Path = [System.Environment]::GetEnvironmentVariable("Path","Machine") + ";" + [System.Environment]::GetEnvironmentVariable("Path","User")
https://stackoverflow.com/questions/17794507/reload-the-path-in-powershell
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$Env:<variable-name> = "<new-value>"
e.g.
$Env:Foo = 'An example'