It has become a lot easier to add and edit the PATH variable in Windows 10 since the first-anniversary update. Earlier I used to use a small program called RapidEE for this.
The steps shown below work somewhat differently in older versions of Windows and in the pre-anniversary update Windows 10.
Go to the start menu and open Advanced System Settings
This opens the System Properties window. The advanced tab should be active by default, if not, click on it to activate it. Then click on the Environment variables button.
Select the variable named Path from either the User variables list or the System variables list. The User variables are visible only to the currently signed-in user, while System variables are visible to all users. Then click the edit button.
This would open the Edit environment variable window. Click on the new button to add a new empty line. Type in or paste in the new path. You can also click the browse button to browse for a folder in the system. Click ok.