What model boat?
Most centerboards are mounted on a pin at the forward end of the trunk. There is a plate on either side of the bottom of the trunk screwed upward into the trunk.
Removing the plates usually reveals some wedges that hold a pin in place.
Remove the screws holding the plates, use a knife to unstick the plates and then the wedges, and slide the centerboard down and out of the trunk. You will need to have the boat up above the trailer for this.
Have it braced or strapped before you release the screws, the centerboards are not light. On the Oday 26, the centerboard weighs around 65 lbs.
There are some pictures covering this operation posted the in this forum. Search on it.
Thanks,
Andrew