Answer
You can edit your ads in the AdManager, which you can access via the settings at the top right of the XING website.
In your Personal account, click on the name of the ad you wish to edit. In the appearing preview of the ad you can now click on Edit ad.
If you use the AdManager Business Account, select the relevant customer and click on your campaign. Here you will find your ad(s) for the campaign and can edit them by clicking on the name of the ad or using the menu button.