If you are calling the WebLogin snippet with a &tpl, that would be the chunk you modify, ex:
[[WebLogin? &tpl=`
FormLogin`]]
in this case you are specifying a chunk to use, and would edit the FormLogin chunk.
If you are calling it simply as:
[[WebLogin]]
you would need to modify the code in the assets/snippets/weblogin/weblogin.inc.php file.
Find this part and modify accordingly:
# Returns Default WebLogin tpl
function getWebLogintpl(){
html code here
}
(dunno if this matters)