Change the Plane Icon
If you would like to change the plane icon at the top of your site in the subscription widget to something else, navigate to Appearance > Editor and then look for the following area of your CSS.
and change the content value to whatever Font Awesome Icon you'd like to use. Click here to see all of the currently available Font Awesome Icons.
Once you find the one you'd like to use, click on that icon. You'll be directed to a page with that icon's information. For example, I'm going to change the airplane to a paper plane. See the highlighted text in the screenshot below.
Copy that code, and replace the above "content" value with it. The end result is the following.
Be sure and keep the / before the code you replaced.