Plane it out on the jointer - this will reduce the thickness.
Thickness it out, concave face down first. This also reduces the thickness.
Cut a curf down the middle on the reverse side, almost all the way through and bend it out.
What's it going to be used for??
I am boxing some radiator pipes in and when i have come to put this piece on the top then run my router along with a flush trimming bit to square the edge up but their is a gap as the cupping is preventing it from lying flat resulting in a gap about 3-4mm all along the front my only other option is to cover it with trim which i don't really want to do.
You can also cut it in half and reglue, if you're careful it will hardly show, then plane flat. Is it finished? If not you could try the lawn trick, place it out in the sun on some grass convex side up, and wait see what happens. It has worked for me, but with panels that have cupped. Keep an eye on it if you do this, sometimes it starts moving quite fast. Once it's straight get it back inside and finish as soon as possible. Another solution, if I understand the situation correctly, would be to just turn it upside-down so the gap doesn't show anymore?