Every Nucleus Card has a corresponding call-to-action that can be customized. This button/call-to-action text appears in a couple of different places.
If your Card is the first card listed in your Homepage List (your Featured Card), the button/call-to-action text will appear in button form as shown below.
If your Card is not the Featured Card on your Homepage, the button/call-to-action text will appear in call-to-action form when a user hovers over the description text of card as shown below.
A Card's button/call-to-action text can be changed within the Nucleus Page editor. Simply hover over web in the top left had corner and click on pages. Then click on the page you want to edit and you should see the option near the top of the page.
When you're ready to to update your Page and commit these changes, click the 'Save Changes' button at the bottom of the page.