You can change your budget on Draft campaigns. Once set live your budget settings are then set and will run until the end date you have selected.

Once you have paid you cannot change the budget and your campaign will run until the end date, or until you decide to cancel the campaign. You can of course cancel any of you campaigns at any time.

If you cancel your campaign or your campaign did not reach the number of impressions that you booked based on your budget, then you will be receive a refund. The refund amount will be calculated by the difference between the booked amount of impressions and the delivered amount of impressions multiplied by the CPM and divided by 1,000.