If you haven’t updated the old Twitter icon to X because you feel a bit stuck with out-of-the-box Font Awesome 4, here’s a CSS hack that should do the trick. Enter in Settings > Code > CSS to make sure it functions across the whole site.
.fa.fa-twitter{
font-family:sans-serif;
}
.fa.fa-twitter::before{
content:"𝕏";
font-size:1.4em;
}