Hi!
Go to:
Configuration > System > Profiles > Wireless LAN > Virtual AP > YourVAP > RF
and choose your band.
Basically it is the same as under 6.4 / 6.5 but the profiles menu moved to configuration - system.
You also can click on your user name (right hand side on the top) go to preferences and select advanced view (or like that).
Now you should also see a profiles link on the right side of the AOS 8 style config menus.
If you operate a Mobility Master deployment, change this setting under the managed network node.