Hi there, just remove the centre piece of floor, the handle for hi/low will just slip off, you will see a broken piece of small pin that locks the handle in place. Remove these, refit the handle and fit a new 5mm (from memory) roll pin. Reassemble, and shift the hi/lo a bit more gently!! (I fell in the same trap, was shifting it with my foot!!)
Cheers,
Aaron.