You can flatten them out by taking a hair dryer to the leather, but they may come back after sitting in the seat again. The leather has been stretched, you can shrink it with heat from a hair dryer, just run it over the affected area back and forth and you will gradually see them shrink. One thing you can do to avoid those stretch marks is to never sit in the seat on your knees, that will stretch the leather very easily as it puts too much pressure on the leather, sometimes people do that when working backwards towards the rear window, perhaps cleaning it or trying to service the softop etc.