To begin with, please keep in mind that the call to action button is just an image (which you cannot edit from within the builder). You can only delete and replace it with another image that resembles a button.
Please note that the replacement image is not available from the builder. You will have to create your own image (button) using a third-party software/app.
Here are the steps on how to replace it:
1. Hover your mouse over the image and an Edit button will show up. Click the Edit button for the editing window to pop up.
2. Click Delete Image at the bottom of the editing window.
3. Once the image is deleted, you are now ready to upload a new button. Click in the column where you want the image inserted (doing this will also make the editing bar appear on top of the column). Click on the insert image icon and upload the image from your files.
4. Once the image is uploaded, you can activate it to work as a button from the editing window. Just open the editing window again and look for the Link: option. You have the option to link the button to an external URL or to a section on your page.