https://serverfault.com/questions/33681/how-can-i-modify-a-user-s-path-environment-variable-without-logging-out

My Computer / Properties / Advanced / Environment Variables. Changes there take effect immediately except for already open command prompt windows. No need to log out and back in.

NOTE: If you don’t have access to that window, you might be able to get it by running rundll32 sysdm.cpl,EditEnvironmentVariables from the Run window or command-line.