What VLAN that SSID is tied to is located in the Virtual AP profile. On the controller, you can create a VLAN, assign it to a physical port on the controller as an access port or as part of an existing trunk and change the Virtual AP to that VLAN.
For example if I wanted to switch an SSID to vlan 8, I would do this:
config t
vlan 8
interface gigabitethernet x,y
switchport mode access
switchport access vlan 8
I would then go into the GUI, and find the Virtual AP that corresponds to the one I want and change it to 8...