The update doesn’t install itself. The safest order of things to do is this:
- Make sure SketchUp and LayOut and Style Builder are all closed.
- Go into Applications and drag the current SketchUp 2021 folder to Trash
- In the disk image window you show in your screenshot, drag the SketchUp 2021 folder onto the Applications alias
- When the copying is finish, eject the disk image (you can right-click in an empty part of the window and eject from the contextual menu, or use the File menu).
- Open SketchUp from inside Applications, and inside the new SketchUp 2021 folder.
In the SketchUp menu, About Sketchup, it should show this version after those steps: