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. If this is what you want to do, remember to let Worldpackers know when you have cancelled so that we can add credits to your wallet and you don't have to pay the fee again!