Power off the PC and unplug it
Remove the CPU cooler
Use a dry lint-free cloth or coffee filter to wipe off the bulk of the paste
Apply 90%+ isopropyl alcohol to a lint-free cloth, cotton swab, or coffee filter
Gently wipe the CPU surface until all paste is removed
Clean any remaining residue from the edges and corners with a cotton swab lightly dampened with alcohol
Let the CPU dry completely
Inspect the CPU and repeat if needed
Reapply fresh thermal paste before reinstalling the cooler