DIAGNOSIS The folding mechanism has two locking knobs, and if you don't release them in the right sequence, the handle binds up. It's not broken, just finicky.
FIX Stand behind the mower. First, loosen (turn counter-clockwise) the lower knob on each side of the handle, near the deck. Don't remove them, just loosen them a few turns until they're free. Then, loosen the upper knobs. Now you should be able to gently fold the lower handle section upward. Once it's up, you can swing the upper handle down to the side.
IF THAT DIDN'T WORK If it's still stuck, check that the mower's height adjustment levers aren't catching on the handle during the fold. Sometimes you need to set all four wheels to the same middle height position to get the clearance you need.
HEADS UP When unfolding for use, reverse the process: attach the upper handle first and tighten those knobs, then position the lower handle and tighten its knobs last. If you don't fully tighten all four, the handle will wobble dangerously while mowing.