Motorcycle Repair: 67-68 ct90 KO, centrifugal clutch, changing oil

Question
I just picked up a CT. By checking the numbers, it shows as a 67 or early 68. Never owning one and not riding a bike in years, IO assumed that the left handle control was the clutch. By piecing a few answers together, and in braking, it feels like the pedal brake and the left lever are tied together. Is this correct? If so, how do you shift? Do you just let off the gas and change to the next higher or lower gear?
I am having trouble finding info. It's a nice little bike and I would like to restore it to original which won't take too much.
Can you also suggest a manual? It seems to slip out of gear once in a while. I will be changing oil that I am told may help as the bike has been in a guy's garage for several years.

Answer
You have a classic bike there.  Those were pretty neat for that period.  Yes, the clutch is automatic. There's a lever inside the clutch case that disengages the clutch as you press on the shift lever.  The clutch is a centrifugal clutch and engages once you start to accelerate with the throttle.

Regards
Rich