Usually it presses out in one direction, usually the boot side down, sometimes there is a circlip retaining it, sometimes under the boot itself. To detach it from the bottom, if it has a castellated (castle) nut, it is drawn in through tightening this nut, you loosen it and hit the side with a big hammer to temporarily deform the outer taper, breaking the tapered part of the ball joint free. Sometimes I just loosen the nut (to protect the threads) and hit the nut until it breaks free.
It depends on what side you're trying to loosen.
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