How to change banner image in Dandelion theme?

I am trying to help a friend update her site and I cannot figure out how to replace the current banner image. She is using the Dandelion theme.

I am not that familiar with how WordPress works – do I need to have it installed on my computer in order to make these changes, or can I just login in and edit it somewhere? I’ve logged in and looked everywhere and cannot figure out how to do it. Help!