|If you have disturbed the heatsink then you must clean off old thermal paste and apply new. There are hundreds of web pages showing you how to do this - just Google. But if you haven't disturbed the heatsink then there is no need to change the thermal compound; in fact you may do more harm than good if you don't do it properly.|
Whatever, you don't need another coat - too much thermal paste is as bad as too little so there's no question of "topping it up". If you do replace it be sure to thoroughly clean the cpu and heatsink before applying it (again Google will tell you all you need to know).
Overheating is more likely to be caused by a dust build-up in the heatsink, or just general dust build up in the computer. Blow it out with compressed air. And clean any filters.