System and Security under
System and Security.
Advanced system settings available on the left panel; a dialog box called
System Properties will pop up.
Environment Variables... available on the bottom-right corner of the dialog box; a dialog box called
Environment Variables will pop up.
NB: There are 2 sets of variables; user variables that are defined only for the logged-in user and system variables that are defined system-wide.
Edit for either user variables or system variables to add or modify variables in either section respectively.