Can I change the itinerary to stay extra days in a place? Can I change the village where I stay on the trek?

Yes, once you have set the route (create itinerary) then you can add extra days to locations that have places to stay. You can also change the place you are staying. To change the overnight stay from one village to another, first add an overnight to the place you want to stay then remove it from the place you no longer want to stay. There is a minimum number of nights required for a given trek, to ensure proper acclimatization, so you must add a night before you remove one. You may also change the itinerary while trekking, but you will need to pay the guide and porter (if you hired one) for any extra days beyond what you booked.