My check engine light came on just days after I had the oil changed. I took
it back to the local service station when I had the oil changed. They plug
in a decoder and told me that I needed to take it to a muffler shop. I
took it to a muffler shop they told me there was nothing wrong with my
muffler or any of its componets. Is there some way to reset the check
engine light or should I be alarmed and take it some where to have it
really looked at? The car seems to run fine. 2002 Chev Malibu LS

The people who plugged the scanning tool should have told you what the
error code actually is. The people who actually fix the car (if any
fixing is required) should be the one reseting the error code.

There is no way to know how critical the problem is when the "check
engine light" comes up until someone competent has given you enough
information. If you read your user manual, it'll say something like:
"go to the service department immediatly, do not drive the car"

Part of the exhaust system is all those fancy o2 sensors and emission
control system (cat converter and other things) the car computer can
detect that something is wrong even when "it looks ok to me"

Have it checked by a service dept and tell you what exactly is wrong.

Take it to another shop, ask family / friends who they use. The place where
you went is totally incompetent if they gave you that line of bull and
didn't reset the CEL. Stay away from that place.
BTW a loose gas cap will turn on the CEL , it will reset after 25-50 full
run cycles.