You can add it to your ~.profile directory.
IF you do not see the hidden files, tap ctrl+h to reveal them in your file manager at Home Directory.
Or for the Environment file:
/etc/environment is in Root.
You can choose your method.
Terminal:
sudo nano /etc/environment
Arrow key down to the bottom (You won't have to go far) and add the line there.
Tap ctrl+x to exit the editor, then the y key to say yes to save, then the enter key to save current configuration.
OR GUI:
In terminal elevate to root:
sudo -i
In the elevated prompt, launch Files
nautilus
Navigate to "Other locations" in left pane, then "Computer" in the right pane.
Then double click the etc directory, then right click the Environment file and select Open With > Text Editor.
Add the line - be sure to click the save button.
Once the file is edited, reboot to test.