When in the Dashboard, on the side menu, click on THEME icon. From here you will see a preview of your website with the current or newly replaced theme. Next to that there are three panels, MY THEME, THEMES, and PRESETS.
To select a new theme, click on one of the thumbnails displayed in the THEME panel. Clicking on the arrows will allow you to view the next or previous batch of themes.
Once you have you chosen theme click on the SAVE THEME button under the PRESETS panel.