Unfortunately, you're unable to directly change the stats in the current version of the game, sorry! A makeshift way to do this is to click on "Open lock menu" in the bottom left and click "Lock All", then click on the "!" buttons near each stat you want to change to unlock them. If you close the lock menu and press any button to reroll, now everything will stay the same except for the stat or stats you unlocked. Keep rerolling until one of the stats you unlocked is the rating that you want it to be and then open the Lock Menu again to lock it. You can keep doing this until all of the stats are locked on what you want, then you can press shift to hide the UI and take a screenshot. Yes, this is very tedious but it's sadly the only way to manually control the stats currently.
I'm sorry that I haven't included this feature in the game and I know that it's highly requested but frankly, I built stats system very poorly. I've tried, but I'm just not a good enough programmer to figure out how to implement a system that lets you directly change the stats working with this current code. I might include this feature in a future update but as of right now I have no plans to update this game any more.