How to separate the heat sink?

I bought Up-board, Up-core, and case.

The board is equipped with a heat sink. So I could not mount the board in the case.
(The case has a block sink to replace the heat sink.)

(I can remove the block sink, but the case is useless.)

So I want to remove the heat sink, but I do not know how to remove it.
Forced use of my hands?

used Google translate. (Some words may be misleading.)