There's a little button where the hose/handle fits into the metal tube. Keep the button pushed in, and pull the hose/handle out of the tube. You can also just pull the metal tube up and free from the top of the cleaner.
Ah! Ask someone reasonably skilled in DIY to have a look at it. Otherwise, ask one of those small independent back-street vacuum cleaner/washing machine repair shops to do it - it should only cost a couple of quid if you've already got the new hose.