My default editor is pico at my server. I use Bash and Linux.
I tried to change Vim to be my default editor unsuccessfully by
echo vim > $EDITOR
You can use the git config option core.editor to set the editor of your liking, eg nano
$ git config [--global] core.editor "nano"
You can also change this by editing the .gitconfig file in your home directory (global) or git repo (create it if it doesn't exist) if you don't have shell access:
...
[user]
name = Your Name
email = your@email.address
[core]
editor = nano
...