how to change "Facebook Login" button

In short, there is no way. That button is loaded from Facebook's server and isn't changeable. They do this to prevent you using misleading buttons to falsely get people to register to your site through Facebook (or worse).
wiki.developers.facebook.com/index.php/Fb:login-button

The text of the button can be changed (as per the URL above). If you're interested in knowing how to do that, just ask and we'll guide you through it.

To change the text, you'll need to modify the JFBCLogin module's template slightly.

Edit the following file: /modules/mod_jfbclogin/tmpl/default.php
Find the text below at line 44:
<fb:login-button onlogin="javascript:fb_login_button_click();"></fb:login-button>
and change it to:
<fb:login-button v="2" onlogin="javascript:fb_login_button_click();">YOUR BUTTON TEXT HERE</fb:login-button>

The 2 main changes are adding 'v="2"' to the fb:login-button tag and then adding your text in between the opening and closing tags.

Hope this helps!