If you are not root user then, use following commands:
There are two ways to do it -
1.
sudo vi path_to_file/file_name
Press Esc and then type below respectively
:wq //save and exit :q! //exit without saving
When using nano: after you finish editing press ctrl+x then it will ask save Y/N.
If you want to save press Y, if not press N. And press enter to exit the editor.