I booked for 20 days but can I change the booking on arrival? and just stay 10 days is feasible?

I think not, that you can change before arriving. Maybe you can consult with the hotel by email

Not sure ... in any case warn them and say that if you cancel the reservation jumps and you will have no room ... so you just want to stay 10 days and not 20 days ... in principle it can not to say nothing ... me 8 nights they offered me the lil