Quick summary from memory. I bought this when it was released.
Growing up, protag always liked girly cute stuff, but it all went to his sister. Even his beloved best friend said his sister is cute. Fast forward to present day. He overhears a girl talking about how she found a sex change app, used it with her boyfriend, then they turned back after having sex. Runs into his best friend. He's jealous of how his best friend likes his sister, thinks he's only going to his house to meet her.
Protag likes girly stuff so much that his entire bedroom is full of it. But he won't wear girl clothes because they don't suit him. He uses app, turns into girl by accident. Admits he's jealous of his sister. Turns out his best friend was only meeting his sister to decide on a perfect birthday gift for the protag. Also, protag's boobs are bigger than hers. They talk about how he can enjoy girl stuff now that he's a girl, and make plans to go places. Sex ensues. Protag turns back. They regret not checking the turn back conditions. The end.