I’ve had a little success stretching out a very high, tight sack.
I do a variety of stretches twice a day every day, once after a shower and once before bed.
First, I gently stretch the balls and scrotum. Obviously you want to be careful here. Just grab the whole sack behind the balls and stretch it in various directions.
Second, I grab the sides of the scrotum with both hands (using all fingers), pull it flat horizontally, and stretch side-to-side and up-and-down.
Third, I grab the bottom of the scrotum (thumbs pointing straight down) and stretch straight down.
After a month, I’ve got noticeably more hang (but still not much). I’m going to give it another few months.