If you want to change Crontab file editor either VIM or NANO instead of using the system default editor, you need to use VISUAL environment variable.
Before that, make sure that VIM or NANO installed in your system if you want to use them.
Specify NANO as the editor for crontab file
export VISUAL=nano; crontab -e
Specify VIM as the editor for crontab file
export VISUAL=vim; crontab -e