Follow the following steps:
1. Edit file qa-theme/Donut-theme/qa-donut-layer.php
2. Locate this line: https://github.com/amiyasahu/Donut/blob/23aab78a3837a29b38139af2e61e9c8f7df8c303/qa-theme/Donut-theme/qa-donut-layer.php#L912
3. Replace the text 'heart' with 'star' or any other CSS class from this file: https://github.com/amiyasahu/Donut/blob/23aab78a3837a29b38139af2e61e9c8f7df8c303/qa-theme/Donut-theme/css/font-awesome.css
Here is the cheatsheet for the current font-awesome icons: http://fontawesome.io/cheatsheet. The font awesome version that comes with Donut is 4.2.0 so there might be a few icons that are not available.