How to straighten AC plywood for desktop

I’m building a very simple work-from-home desk, with 3/4" AC fir plywood for the top. The plywood has a concave bow on the top side. How do I straighten this board?

Other details:

• I’ve already applied several coats of polyurethane to the top–perhaps this was a noob mistake to do before straightening.
• I’m planning to use sawhorses as legs, and screw a 2×4 lengthwise to the bottom of the desktop for rigidity.
• Desktop is 72"x24".
• I didn’t finish the bottom. Do I need to do this for moisture reasons?