Like this
HTML
<a href="#" class="btn"><i class="icon-rotate icon-flipped"></i></a>
CSS
.icon-flipped {
transform: scaleX(-1);
-moz-transform: scaleX(-1);
-webkit-transform: scaleX(-1);
-ms-transform: scaleX(-1);
}
OR
http://fortawesome.github.io/Font-Awesome/examples/#rotated-flipped