You can find out what's possible for the different products/license in your My VMware portal.
Select the downgrade license option for the existing license, and the wizard will show you the available options.
In the old days the menu youre looking for was labeld "Actions" or so.... yes its in the upper left when viewing your contracts and contains workflows like combine, divide, upgrade and downgrade licenses. After selecting you get a list of all your product licenses which can be downgraded.
Moderator: Moved to vRealize Operations Manager
Is there a specific reason why you want to downgrade to 6.x?