Use the update-alternatives command.

In a terminal window and type in the following command:

sudo update-alternatives --config editor

Here's an example of what you'll see:

$ sudo update-alternatives ––config editor

There are 5 alternatives which provide `editor'.

  Selection  Alternative
––––––––––––––––––––
           1 /usr/bin/vim
           2 /bin/ed
        *+ 3 /bin/nano
           4 /usr/bin/vim.basic
           5 /usr/bin/vim.tiny
Press enter to keep the default[*], or type selection number: