To start with you don't want to twist the fork by using high torque on the headset while holding the fork by the wheel. Also note that this headset has two locknuts.

My preferred method is to hold the knurled upper race with pump pliers (Channellocks) while using an open end or adjustable wrench on the hex locknut. Protect the race with an old leather belt to avoid marring it.

Once that locknut is off remove spacers and cable holder then use a hook spanner or hammer and punch to loosen the locking ring. Then the cup will be free to take off by hand.