If the chuck has no provision for a wrench, the easiest way to turn it forcefully is by means of a hex key gripped in the chuck jaws by its short end.

If a wrench can be fitted to the spindle, lay the drill flat on the edge of a workbench or sturdy table, with the wrench resting on the table, the chuck overhanging the edge and the hex key pointing slightly upwards from horizontal.

It won't hurt to try if you can release the chuck by pushing down on the hex key by hand - though this will rarely work.

If hand force isn't sufficient, firmly hold down the drill and give the end of the hex wrench a solid whack, preferably with a wooden or plastic hammer. A sharp blow should loosen the chuck instantly.