changing transmission fluid?

2001 grand am se(4 cyl.). There is no dipstick for the tranny so how do you get fluid back in it after dropping the pan? How often do these need to be changed? thanks.

If you have the 4 spd auto. there is a fill cap on the top of the trans. Remove that cap, and then under the car, on the rh side of the trans, near the axle shaft there is a plug, remove that, and fill the trans untill fluid comes out of the hole.