Start with a razor blade between them and cam them apart.
The heat sink compound has become hardened glue.
Clean the heatsink and put spread a dab of heatsink compound on the new CPU so you get a very thin 100% coverage.
Ed Fair
Any advice I give is my best judgement based on my interpretation of the facts you supply. Help increase my knowledge by providing some feedback, good or bad, on any advice I have given.