Yeah, I mean the thermal compound
As for doing that, it should just come off by scraping it with something not so hard, but enough to get it off (the top is only the cover for the chip so should take some force, though you should know how lightly to do it
). Most of it will be on the heatsink anyway which you'll be able to scrape off without damage.
Then just reapply about the same amount (not that thick, and about the size of the CPU chip, on the heatsink itself, and then place the heatsink back on, click it into place, and place the fan back. Sorted
I hope this helps