Hi Mattias -
Unfortunately we don't have a good solution to this problem yet.
You could delete the Deployment Manager database, which would "reset to factory settings", but this would lose all of the information you've entered, including environments, projects and agents you've set up.
If you do want to delete the database:
1) Stop the Deployment Manager service. It should appear in services.msc as "Red Gate Deployment Manager"
2) Delete the directory you have assigned to store Deployment Manager data. The default is %PROGRAMDATA%\\RedGate\\DeploymentManager\\Data, but you can find out your data directory by loading the Deployment Manager server tools application on your DM server and selecting the "data repository" tab.
3) Restart the service, either in services.msc or using the "service" tab of the Deployment Manager server tools application. A new, empty, data
directory will be created on restart, and if you navigate to the DM portal in a web browser, you will be invited to create a new administrator user.