How do I undo the front sprocket nut?

Engage crawl mode or reverse and undo it with an impact wrench. Without one, brace a ratchet against the footpeg and give it a gentle bit of throttle — the Stark will undo it itself.

Usually you'd hold the rear wheel so the chain doesn't turn.

Stark video: replacing the front sprocket, MX 1.2 — https://youtu.be/m_4MPSopNKg