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
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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: