If your worried about contamination, it needs to be cut open and cleaned.
If you just want to get the old fluid out. Drain it into a coffee can or something as described and refill with clean fluid.
It is possible to flush them by drilling and tapping for a 1/8", then pumping fluid through them. When complete you screw a pipe plug into the hole, then weld it. If it were mine and I was worried about some sort of contamination, I'd open it for cleaning.