WBC is normal (6.0), but mono% is too big (13, 4%) while mono count is still upper normal (0.8). Do I need to worry?

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Not to worry. Please rest assured and do not worry. Unless you have symptoms fever, malaise and others, the normal WBC and normal absolute monocyte counts should be a good sign. Fluctuations in blood subset percentages are common and normal; it is a way to say our body is responding well, regardless of age, to any little or more potential infections, tissue cuts or injuries.
Labs.... Hi L.M320, in terms of lab results, it is best to let the doctor that requested them interpret them for you. He will have a better understanding as to the context. Sometimes, an abnormal value is fine and sometimes a normal value is not fine. Understanding the background and context makes all the difference. You should make some time to communicate with your doctor. Hope that helps.
Stop worrying. Modestly elevated monocytes like that basically never mean anything if you feel okay.