Unscrew these 6 screws, after you have taken the optics out of the case. Note that the three motor screws where in place during the photo, but should be removed first

To be able to separate the Sensorbackplate from the rest, you have to unsolder the conenctor

Also disconnect this connector, as well as the connector on the opposite site

Finally you have the sensor with (maybe?) the auto focus lens.

If you have dust on your images, it is not necessarily on the sensor. It is rather unlikely because it is enclosed from the auto focus lens. Maybe you should search on other part of the optics first