Converting from camelcase to _ in emacs

Use the string-inflection package, available on MELPA, or at https://github.com/akicho8/string-inflection.

Useful keyboard shortcuts, copied from https://www.emacswiki.org/emacs/CamelCase :

;; Cycle between snake case, camel case, etc.
(require 'string-inflection)
(global-set-key (kbd "C-c i") 'string-inflection-cycle)
(global-set-key (kbd "C-c C") 'string-inflection-camelcase)        ;; Force to CamelCase
(global-set-key (kbd "C-c L") 'string-inflection-lower-camelcase)  ;; Force to lowerCamelCase
(global-set-key (kbd "C-c J") 'string-inflection-java-style-cycle) ;; Cycle through Java styles