Hi Arthur,
Most likely, it has a threaded spindle nose. If you have both chucks, or a faceplate, just look at the back side of one of them to find out
They are a right hand thread, if it is a threaded mount. This can sometimes get pretty tight. The method to remove, is to first shut off the power to the machine, since you do not want it to start accidentally while you have a wrench on the chuck.
Engage the back gear and the direct drive gear at the same time. This locks the spindle up solid so you have something to work against when you try to loosen the chuck. Take a large wrench and adjust it to fit one of the jaws of the chuck and pull down on it, rotating counterclockwise. As soon as it is loose, it should come off easily.
Take plenty of care of your spindle nose threads and the shoulder at the back of the threads: this is what determines how true your chuck will run.