To make changes to a campaign:
From your Dashboard, click Manage-> Campaigns.
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Hover over this area next to the campaign you want to edit and the buttons will appear. Click the Edit Campaign button.
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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.