Ok, this one was a bit more involved because it uses code :) Short answer: you need to use hook_form_alter() to take control over that particular form. Fear not! I will show you how to find these things and edit them quickly :D
Longer answer: the trick is to know where to find these things, so I spent a few minutes explaining my process for finding what I want to alter.
So that's the video of how I made it happen. Below is the steps I took to alter the user_login form for Commerce Kickstart 2.
STEP BY STEP
1) Find the form id ... I did this by inspecting and looking at the "id" ... for us, it was user_login
2) Search the text of the Drupal site for mention of this form_id
3) Copy function that modifies this form
4) Paste function inside the template.php for your theme (or your module_name.module file for a module)
5) Rename the first part of the function
6) Change html to your heart's content.