How do I update creative for an existing campaign?
From your campaign's Overview, click into the Creative tab at the top. In the top right you'll see the button to Upload Creative.
Existing creative files cannot be swapped out or replaced, but you can upload new files and optionally Deactivate the old creative. Creatives can only use unique names, so if you plan to use the same new for new creative, make sure to change the name of the old creative by clicking "Edit Creative" before Deactivating.
If you are sending triggered mailings for that campaign, please remember to switch out the old creative_id 's to the new one(s) in your integrated workflow.