Go to the Advertiser from the Advertiser dropdown (top right of platform).
Find the campaign with your ad in it (click “All” at the top to see all campaigns for this Advertiser - see article). (Or you can go into the Builder and find your ad build).
Click on the 3 dots to the right of the ad you want to edit:
Select “Edit Ad Unit”:
This will take you into the ad within the Builder.
Right Click on the layer at the bottom of the builder that you want to replace and click “Replace Asset”).
The asset manager screen will appear for you to select your asset (if not in the Asset Manager, click the cloud arrow icon in the bottom left corner of this screen and select the asset from your computer).
Save the Ad.
Go back to the campaign click the Republish icon so the changes automatically update within the current tags (no need to export new tags if this ad unit is already in an existing placement in a campaign):