Hi there, welcome to the forums!
"Flushing" your transmission at this point is a bad idea, the worst being with the "power flush" that many shops tend to do. You can, however, do some drain and fills to slowly replace fluid, with the goal of having a gentle transition to the new fluid.
Remove the drain plug and let it all drain out (you'll probably get about 3.5 quarts). Then fill up the same amount through the dipstick tube. Any good quality Dex III compatible fluid will do. For conventional, I'd recommend Castrol's Dex/Merc, and if you want to go synthetic, go with Mobil 1 Synthetic ATF, very good stuff. Doing one of these "drain and fills" replaces about 1/3 of the fluid. So do 3 or 4 of them, with some driving in between to get all the old stuff out.
There are also write-ups here on how to do a flush, but by using the power on the transmission and therefore reducing the risk of damage that power flushes can do.
Feels free to ask any questions you may have, many people here have a wealth of information concerning these topics!