You can merge scenes from the full chapter view by simply removing the scene break. To remove the scene break, the trash can icon to the top write of the placeholder image in the main chapter Writing section. This will combine the scene with the previous portion of the chapter, it will not delete your content itself.
You can also merge scenes from the left navigation pane by hovering over the scene title and clicking the X that appears. A confirmation window will appear where you can select “Merge Content” to merge the contents of this scene into the following scene.