First, wash and dry it normally, according to the care instructions on the tag. Do this even if it’s clean. You want to iron just after removing it from the dryer.
Polyester must be ironed very carefully! Always iron from the wrong side and always use a pressing cloth so that the iron never comes into contact with the polyester. Keep the iron on the “poly” setting.
For the stubborn, “set-in” creases, use a damp pressing cloth. Be patient and use pressure, but do not increase the heat. Iron from the top to the bottom of the garment in a side to side motion.
If this doesn’t seem to be working, try spraying a solution of white vinegar and water onto the crease/s. (You may want to spot-test this first in an inconspicuous area, if the garment is really old.) Iron with your damp pressing cloth again.
An alternative is to wash or wet the garment and hang it to dry on a clothesline, hoping the weight of the wet garment will help to pull the wrinkles out. Check on this frequently as it dries to be sure it is hanging appropriately so that you do not create more wrinkles. However, these mild wrinkles will be much easier to iron out than creases that have been “set” from packaging or storage.