Cleaning rotary on/off switch assembly

My son-in-law ‘loves’ his Nikon D90. He is a very keen plane spotter and as a result the camera spends much of it’s life outside with a long tele lens hanging on the end of it.

The lens does not come off the camera, so sensor cleanliness is not the issue.

No, it’s the external body… dust under the rotary on off switch for example - this now is becoming more and more difficult to rotate.

Grateful for step-by-step guide on disassembly of this area for cleaning..