Microwaving the pumpkin/squash for 60 to 90 seconds will do wonders for softening the stiff flesh. If you’re still having trouble, microwave it in 30-second increments until it’s soft enough to slice.  And remember to use your largest, sharpest knife! This is a dangerous job for a dull knife. It also helps to slice off the top, then the bottom nub so the pumpkin is on a flat surface. From there, carefully slice the pumpkin in half lengthwise (use a back-and-forth motion, like a saw).