Preschooler stuffy + runny nose. (clear in color)?

Humidifier. Humidifier and plenty of liquids. If last longer than a week see your doctor otherwise without any other symptom can treat at home.
Not much to do. Clear runny nose at this time of year is very likely to be just plain discharge related to cold weather or a mild common cold. Mostly, this bothers you more than the child and i don't recommend parents do much. No cold meds at this age. Many times having the child sit and read while in a steamy shower can help the snot flow out before sleep, etc...