[header] How to thicken frosting [title] Add more powdered sugar. [step] Most frostings contain powdered sugar, and the most common way to thicken a runny frosting is by gradually mixing in a bit more powdered sugar to offset the liquid ingredients. [substeps] Only add the sugar a little at a time. 
Sprinkle an additional 1 to 2 tbsp (15 to 30 ml) into the frosting and mix well. If you add too much at once, the frosting will become too sweet and may become too thick.