You might find that Ranger uses Nano as editor when you open text files. It does this if the
EDITOR environment variable is not set.
To make Ranger use a different editor to open text files, just set this environment variable.
For example, in the Fish shell, I set the editor to
vim by adding a line to
set -x EDITOR vim
Tried with: Ranger 1.6.1 and Ubuntu 14.04