Hi,
You should be able to do it with font awesome and custom CSS code:
stackoverflow.com/questions/14736496/use...awesome-icons-in-css
Now, for a simpler solution, you might want to directly modify the HTML of the filter button in the product / filter view file via the menu Display>Views:
<div class="hikashop_filter_button_inside" style="float:left; margin-right:10px;">
<input type="submit" id="hikashop_filter_button_<?php echo $this->params->get('main_div_name'); ?>" class="<?php echo $this->config->get('css_button', 'hikabtn'); ?>" <?php echo $attributes; ?> onclick="<?php echo $js; ?>" value="<?php echo JText::_('FILTER'); ?>" />
</div>