You can exchange the soul stones of 5-star minions for the soul stones of other minions. You can't get soul stones for minions that are already 5 stars or for which you have enough soul stones to promote it to 5 stars.
Every day at 05:00 the soul stones you can get will refresh.
Your first exchange rate of the day is 3-to-1. The more you exchange, the more expensive it becomes. The max exchange rate is 7-to-1. You can refresh your exchange attempts, but the exchange rate will remain the same that day.
You can get lucky when exchanging soul stones. A critical exchange gives you 2 soul stones. A super critical exchange gives you 5 soul stones.
You can find soul stones that are on discount. These discounted soul stones have a fixed exchange rate of 3-to-1. Refreshing the shop may make these discounted soul stones disappear, so be careful!