Turn off the computer and unplug it
Remove the CPU cooler
Clean the old thermal paste from the CPU and cooler base with isopropyl alcohol and a lint-free cloth
Let both surfaces dry completely
Apply a small amount of thermal paste to the center of the CPU
Reinstall the cooler straight down onto the CPU
Tighten the cooler evenly
Reconnect the cooler fan cable
Power on the computer and check temperatures