With the board out of the case, you just carefully squeeze the plastic posts together that hold the HSF on the back of the board, and push them through.
EDIT:, Unless someone knows a different way, you will have to remove the board fron the case to do this, unless you plan on trying to pull the plastic posts through the front with pliers, (not advised) and re-attach the HSF with Arctic Silver thermal adhesive, or frag tape. Unless you have some extra posts around.