Within the 'Ad manager' tab of self-serve, you can change details of your live campaigns. If you select the ‘Edit Campaign’ button under the 'View more' column, then you can amend the end date, total budget, add/remove a cap, or adjust your CPC.
Any time you make an adjustment to your campaign you will need to “Save & submit for approval” again, if you “Save” your campaign it will be paused.