Can't really comment on how to change the belts.
I have tried all manner of interior cleaners from A. G. and Megs., none seem to clean the belts at all.
So I tried Persil Washing Liquid with boiling water, the stuff you pour into a ball and place in the middle of your clothes in the washing machine.
I used a stiff brush to lather and a towel to dry, and now they are clean.
I pulled the belt all the way out, and used clothes pegs to stop it retracting, after wiping I left the belt on the towel over night, removing the pegs in the morning.