change login text

Customizing_the_Login_Form#Make_a_Custom_Login_Page may provide you some clue on how to achieve what you intend to do. There are a couple of links to resources that explain it in depth too.

I’m not sure that’s the only place though.
I looked in the general-template.php file and this line of code exists:
‘label_username’ => __( ‘Username’ ),

However, the actual lable on the login page says “Username or email”