The HTML5UP templates are using Awsome fonts
On this page you find all the icons you can use for your buttons. There is a layout for buttons and if you want to change icon, you need to edit this button layout.
The template is h5up01_button. In this example it has four CSS classes concatanated like this, class="button button-icon fa fa-hand-o-left" and you change the name of the last class (in red) to any of the other class names displayed on this page.
Some of the buttons don't have any icons. In those cases you need to add a fith class name. For example the fa-spin has a icon, and it has that because there are a fith class concatanated.
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Like this class="button button-icon fa fa-spin fa-upload"