If you want to change the dates of your travels before you confirmed your trip, all you need to do is ask the host to change the pre-approval dates to the newly arranged ones.
If your host agrees, he/she will send you a new pre approval with the dates you need, which you can then confirm :D
If, however, you have already confirmed your trip, you have two options:
1) You can simply talk with your host and agree upon some new dates and on your own terms. Keep in mind that you will not be receiving a new Confirmation Voucher if you do so.
2) You can CANCEL your trip, re-apply, get re-pre-approved, and RE-CONFIRM this new trip.