I have done this on my 5D III

I found my 5D III has few spot after a day birding trip.  Change the setting to Bulb, and then mirror lock. Carefully using can of compress air to blow the dust away. 

First, give some practice with the compress air.  Don't shake the can and don't blow too long nor press the triggle to much. Always have the air can vertical. a squak of gental air is all you need.

Good luck!