Go to your shopfront then click on Manage My Shop in the top right directly above your banner. That puts you into Edit mode. On the Edit page, you have two links immediately below your circular logo - Stage Your Shop and Tell Your Story. The default is Stage Your Shop. You need to click on Tell Your Story to get to the page for editing your About section. On this page, you'll see the pen icon to the right of your video which you click on to edit the video.
