I noted some people saying they were confused by nano. Here are basic
instructions to get you far enough that you can install your editor of
choice. I am deliberately leaving out things like cutting and pasting.
If your goal is just to get a different editor installed, these
instructions should suffice.

1) To edit a file, type:

nano /etc/apt/sources.list

2) Use cursor keys to navigate.

3) Typing inserts text. The delete and backspace keys work as
expected.

4) Ctrl-O saves the file. Nano's menu at the bottom of the screen
calls this "Write Out". You will have the option to change the name
of the file. On my system, there is no blinking cursor. But the
cursor is in fact at the end of the filename. Use your cursor keys
or the backspace key and you will see it.

5) Ctrl-X exits. If there are unsaved changes, it will prompt you
to save by asking "Save modified buffer?".