Your ceiling diffuser is quite easy to remove and clean. Please note that older Solatube Daylighting Systems (Installed before the year 2000) have twist-lock diffusers for the 10 in/250 mm & 14 in/350 mm. For these older units the diffuser twists off counter clockwise when looking up at the diffuser.
If you have a unit that was installed between 2000 & 2007, to remove the diffuser simply pry down one edge of the white trim ring using something flat such as a butter knife. Continue to work your way around the ring until you have a 1/4" (7mm) or so gap all the way around. Then simply grab onto the edges and pull straight down. The diffuser lens can be cleaned with soap and water. To re- install the diffuser position the 4 zip style ties into the 4 holes on the ceiling ring and push up until the diffuser is flush with the ceiling.
If you have a unit that was installed between 2007 & 2010, simply pull the diffuser down.
If you have a diffuser that was purchased after September 2010, you'll need to twist off the diffuser counter-clockwise.