To make changes to a campaign:
From your Dashboard, click Manage-> Campaigns.
Hover over this area next to the campaign you want to edit and the buttons will appear. Click the Edit Campaign button.
Make your desired changes to Basic Campaign Info, Campaign Targeting, Conversion Goals, Campaign ads and/or Campaign Budget & Bidding.
Click the green "Save Changes" button at the bottom. All done!
Note: If you increase the budget on a campaign, you will be charged the difference between the new budget amount minus the remaining funds in the account. If you lower the budget on a campaign, the campaign will not charge you again until the remaining balance drops below your budget amount.