This document will guide you through the process of setting a default editor. There are two safe ways. Both are covered here.
You will need to put a variable, for example. To use nano when you are going to edit a crontab, do the following:
EDITOR=nano crontab -e
This will use nano only for that execution.
You will need to do the following:
alias crontab='EDITOR=nano /usr/bin/crontab'
This will work on your current session. If you want to make it permanent, you can put this line in your bash files