Just follow the steps
STEP 1: Replace the old line by the new one (see image below).
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.3.1/css/all.css" integrity="sha384-mzrmE5qonljUremFsqc01SB46JvROS7bZs3IO2EmfFsd15uHvIt+Y8vEf7N7fWAU" crossorigin="anonymous">
STEP 2: Search the new icon you want to use FontAwesome.
STEP 3: Select your favorite icon, copy and paste the new prefix in Pingendo.
NOTE: The fa prefix has been deprecated in version 5. The new default is the fas solid style and the fab style for brands.