reCAPTCHA banned. Solution found, needs a little help

[Gaius Kong]

reCAPTCHA banned in China. Solution: 

You may try recaptcha.net. It's offical and accessible from China. Just change https://www.google.com/recaptcha/api.js?render=explicit to https://recaptcha.net/recaptcha/api.js?render=explicit in front end, and https://www.google.com/recaptcha/api/siteverify to https://recaptcha.net/recaptcha/api/siteverify in back end, and it should work as expected.

I want to replace ALL of them, and there are twenty of them in different files, as I see them using this command line:

find . -type f -exec grep "www.google.com/recaptcha/api.js" '{}' \; -print

Help needed: what is the proper command line using find, sed or perl to replace string1 with string2 in all subfolders and files, especially when these strings have "/" in them?