You can upgrade contacts to responders, and responders to users by selecting the person you want to upgrade, as shown in the image below.
In this example, click on "Upgrade to user"
This tab will then appear — click "Upgrade" to upgrade the responder to user.
If would downgrade from for example user to responder, this tab would appear:
The order for upgrade: contact-> responder->user
The order for downgrade: user->responder->contact