if you can't or don't want to install nano for whatever reason you still can use vi which is preinstalled on every linux machine. Please note that vi is one of the oldest editors ever. it was written when computers still had smaller keyboards and were lacking arrow keys, number pads and the like. Because of this vi is a bit funky to use for beginners but very powerfull once you get used to it. Here is a small how to how to use it.

Open the file you want to edit (again, get superuser rights before that if you need to edit system files)
Now you can see the contets of the file bu you cannot edit it yet because vi is in navigation mode. Press "i" to switch to insert/edit mode. Now you can edit the contents. After you are done, press Esc to leave insert/edit mode. Now type ":" to enter command mode. Type "w" to write (save) your modifications and then "q" to quit vi. If you have messed up the file and dont want to save your mdifications type "!" instead of "w". The line on the bottom should show ":wq" if you want to save and quit or ":q!" to just exit without saving your changes. Hit enter to actually execute the command. You can also use only ":w" to write the changes without exiting. Google for vi reference list for a complete list of vi commands. They are also available in form of mouse pads or mugs