Font Awesome has NuGet packages named FontAwesome.UWP and FontAwesome.WPF. Just download one of this.
If you will use a icon import follow namespace into your XAML code:
xmlns:fa="http://schemas.fontawesome.io/icons/"
Use it into your button like this:
<Button x:Name="btnButton">
<Button.Content>
<fa:ImageAwesome Icon="LongArrowLeft"/>
</Button.Content>
</Button>
And finally in your C# code behind:
using FontAwesome.WPF; // on the top of the code
btnButton.Content = FontAwesomeIcon.LongArrowRight;