REG QUERY HKEY_CURRENT_USER\Environment
REG QUERY HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\"Session Manager"\Environment
or
REG QUERY HKCU\Environment
REG QUERY HKLM\SYSTEM\CurrentControlSet\Control\"Session Manager"\Environment
You can also launch System Properties, choose the Advanced tab and click Environment Variables...
sysdm.cpl
Or launch the Environment Variables screen directly using...
rundll32 sysdm.cpl,EditEnvironmentVariables
SETX VARIABLE "value"
Alternatively use
regedit
or
rundll32 sysdm.cpl,EditEnvironmentVariables
or, to also be able to Edit the System Environment Variables...
start-process rundll32 sysdm.cpl,EditEnvironmentVariables -verb RunAS
SETX VARIABLE ""
REG DELETE HKCU\Environment /V VARIABLE /F