Changing the actual environment variables can be done by using the env: namespace / drive info. For example this code will update the path environment variable
$env:Path = "SomeRandomPath";
There are ways to make environment settings permanent but if you are only using them from PowerShell, it's probably a lot better to use your profile to initiate the settings. On startup, powershell will run any .ps1 files it finds in the WindowsPowerShell directory under my documents. Typically you have a profile.ps1 file already there. The path on my computer is
c:\Users\JaredPar\Documents\WindowsPowerShell\profile.ps1