Rough Synopsis: black hair guy click on a moble game advert and ends up download an app that changes reality. He turns himself into a slut then hits save that updates reality so he was always a girl. Then he uses the app again to turn into the slutty childhood friend of the other guy. Sex ensues. Ends with 12 hours left until reality changes become permanent
You can see the ending because they have wedding rings on in the first page. So at some point after the last page, she decided to rewrite reality and marry him.