Don't know why you want to do this but, remove as much as you can with a SOFT tissue from both the CPU and heatsink. It is important to avoid scratching any surface. Then wipe over with a soft tissue soaked in petroleum spirit (lighter fuel, "Ronsonol"). Don't dip into your car petrol tank as this contains all sorts of additives which you don't want.
The "pits" that the compound fills (thus ensuring better heat transfer) are microscopic and your current grease will have filled them adequately. If you remove this grease there WILL be traces left in these pits and any new compound will not be able to fill them and so be pretty much redundant.
If you wish to remove the grease I hove found that white spirit does the job well BUT - it will leave tiny (microscopic) amounts of oil in the very same pits that you want the new compound to fill and again, render the new compound useless!
I cannot find it now, but somewhere on this site is an excellent link to the Artic Silver website that explains all the above in detail.
You are better off leaving as is I can assure you.
I use Isopropanol (Propan-2-OL in new money). Its available for a couple of quid from a Chemist. It evaporates completely so doesn't leave minute particles of oil which white spirit will as per Smiffy99's post.
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