There is a few software that will use the editor command to find out what text editor to use. Example commands will be dch to add a new .deb changelog entry, revision control softwares when prompting for commit a message …
There is basically 2 ways for changing the default editor:

1. System Wide
Run Terminal and type:
$ sudo update-alternatives –config editor
And then choose whichever editor you want to be default.

2. User Level
As a user, you cannot change the setting for the whole system, but you can add an alias for editor to let say vim.
Open and edit ~/.bashrc and add:
alias editor=vim
Next time you will open a bash prompt, your default editor will be vim.