The usual story for stuck steering clutches is soak them in some kind of solvent e.g. acetone/ATF mix for a while, and/or tie the levers back and drive around riding the brakes til they're smoking hot hoping that the heat helps them release, and/or bumping into an immovable object with the blade if it has one, or towing a very heavy load (like a D9 with its brakes applied). If all else fails, disassemble them - that is guaranteed to work but typically is the most time-consuming and expensive. Results have been all over the map as to what works for different tractors and it likely comes down to how stuck they are. In my 5U's case, I had to dismantle the clutches with a gas axe that's how stuck they were. Other folks have just had to pull the levers and jab the brakes a couple of times. Hard to say with yours unless you know how long it's been since they were free and what the moisture conditions in the clutch compartments have been over that time. One other thing to check is whether the clutches are correctly adjusted because that has foiled a person once or twice too.
With all that written, your sequence should probably be levers back and ride/jab brakes, then soak for a week or two and try again, then disassemble

Yours is 20 later than mine - 5U13753