1) Easter Week section actually used the Recent Posts element in the builder, which means it retrieves the latest posts of your WordPress installation. So if you go to WordPress Dashboard > Posts you will see the post with the proper Featured Images. As soon as you change them or you add new posts there that section will be updated automatically and show your posts.
2) Band Image: you need to go to the section which contains the columns of the Our Mission and the Youtube video and click on the Magnifying glass icon of that section and you will see the section which contains background and the image:
3) Youtube Video: That section is using the Classic Embedded Video element and the link to the Youtube video which you can change the options of that element.
I suggest that you check the articles under the Cornerstone/Content Builder and the Elements section of our knowledgebase to get familiar with the ins and outs of our builder.