Convert hyphen to underscore in permalinks

I would like to use underscore in my permalinks instead of hyphen.

Current permalink:

www.example.com/2013/01/hello-this-is-a-test-post/

Desired permalink

www.example.com/2013/01/hello_this_is_a_test_post/

I have tried some solutions mentioned here in stacexchange. But they were not working.

it can be done easily without any codes or plugin just go to pages dashboard, click Quick Edit” which url you want to change. Now add underscore. Update and done