I have a check engine light and code P0440. How can I clear the code?

Question

Code has popped up 3 times. First time I took the car to the local oil express shop. They determined the code, looked around, and found that there was a hose to the intake manifold disconnected. He put the hose back on, cleared the code and all seemed well. 2nd time it happened at virtually the same location on the same route I was driving from my home. I checked the hose, and it was connected. Took it back to the oil express shop, and they looked around and saw nothing in particular. Suspected that it might be the fuel filler cap, but could be any number of items in the evap control system. He gave me the name of an auto repair shop that "smoke tests" the system. He also reset the code and told me to put the cap on tight in the future. I hadn't gotten around to refilling my tank, and so the cap was not "extra tight". Sure enough the code came back up, at virtually the same location on the same route I was driving from my home. So time I inspected the cap more closely - it seemed fine, but I noticed some surface rust and some roughness around the filler neck. So I "steel wooled the rough surfaces smooth, cleaned up the area with a small vacuum, and put the cap on securely (waiting for clicks). Now I'd like to clear the code and try again. How do I clear the code?

Thanks.