Assuming you actually have a 9V AC output, and you want a 6V AC supply, then you should use a power transformer with a 3:2 winding ratio. Supply 9V AC to the "3" side, receive 6V AC on the "2" side. Magic! :-)
Btw: Some people call anything that plugs into the wall an "AC" adapter, because it adapts the "AC" from the wall, even if it generates DC. If the supply generates 9V DC, and you want 6V DC, then the best way to do that is with a switching DC DC converter (step-down converter, typically "buck" converter.) Pololu sells some decent ones. Hobby stores also sell them as "UBECs."