You can remove the hose clamps on the site tube. Lower part to drain, upper to fill. If you do this I suggest replacing the tube and make sure to use the same length as existing. Be sure to mark the full level (should be 2 quarts-but double check) with a tie wrap/cable tie.
The other way is keep the tube in place and remove the brass parts that the tube goes into. You will need two wrenches.
In either method you will need a hand pump to fill the new fluid at the top where you removed the upper part of the tube. It will likely be too difficult to use a funnel.
I use the Torco fluid but you can use alternatives specified by Atlas.