Editing Sign Up Button on Form Used on Landing Page

Hi! 

I'm building a Landing Page for a free book download. 

I'm able to edit the form just fine on the Landing Page, but I wanted to change the button text from "Sign Up" to "Download." 

When I change it, it shortens the word to "Dow." 

I also changed the color of the button and that works fine. 

How can I get my button to say "Download" in its entirety? 

Thank you!