The first step is to double click on a thumbnail to open the editing options. Several tools for editing and some filters are provided. Among the several buttons, you shall get the following:
The Cancel button removes one by one changes applied. The Settings tab contains several sliders to change the lights, shadows and color temperature. Finally, the Effects tab offers several filters such as Sepia mode or artistic blur.
To validate a modification, just click on Store option found in the lower panel.