This is how it worked for me for dnn 6.1. I'll break it down for novices (as I am one).

To start with, DNN was installed in www.domain/dnn.com with a child portal at www.domain.com/dnn/child.
I then moved the installation up to www.domain.com, and the child portal up to www.domain.com/child, eliminating /dnn/ altogether.

1. First check with your host provider that they allow a root installation of DNN.

2. Sign in to your installation at www.domain.com/dnn and go to Admin: Site Settings: Advanced Settings: Site Aliases. There you add the alias www.domain.com to your main installation. You must do this BEFORE you move the installation.

3. Now back up www.domain/dnn AS WELL AS your database.

4. Copy all the files in www.domain.com/dnn to www.domain.com. (You can use ftp.) MAKE SURE that you delete the www.domain.com/dnn directory after it has been copied to www.domain.com. (If you go to the next step without deleting the /dnn installation you will have to go back to scratch.)

5. Open up www.domain.com in your browser and sign into the dnn site at this new location.

6. Go to Admin: Site Settings: Advanced Settings: Site Aliases and set the Default Alias for this site to www.domain.com.

7. Then delete the old alias www.domain.com/dnn.

8. At the same time, still working within the parent www.domain.com portal, and still at Admin: Site Settings: Advanced Settings: Site Aliases, create a new Alias www.domain.com/child for the CHILD portal.

9. Then select this as the Default Alias for the child portal, and

10. Delete the old child portal alias www.domain.com/dnn/child.

11. Finally you will need to edit internal links to remove /dnn/.

Some of the image paths may also need to be adjusted.

That's how it all worked for me.