To edit your budget, you must be in the Pay Per Click Dashboard. You can get there from the Jump To menu in top left corner and selecting, "Ad Campaigns."
From there you need select the Campaigns tab (it should already be selected by default).
Here you can edit the budget that you have allocated to each campaign by clicking on the blue pencil next to each budgeted amount. See screenshot below.
Note that changing or editing your keywords and keyword bids will not effect the amount of allocated budget to each campaign.