You can normally set local environment variables in two ways:
Option 1:
1-Click on Start, search for "Edit environment variables for your account" and click on that
2-This option will open a window like you sent on the screenshot where you can only change the user variables
Option 2:
1-Right-click on Start then click on Run
2-Type: sysdm.cpl and click on OK
3-Click on the "Advanced" tab, then click on the "Environment Variables..." button at the bottom
4-This option will open a window where you can change the user variables and the system variables
Try to access the environment variables using option 2 and check if you are able to edit the system variables.