This kind of future is unheard of !

Kayo Hanazono, a second-year high school student, has been hiding her feelings for her childhood friend Shinnosuke, and she thought she'd be safe doing so. But then Kayo's 30-year-old self appears from the future! She is told that Shinnosuke is getting married to another girl he has been dating since high school, and that Kayo is still single at 30. There is no time like the present to change the future!! The days of high school Kayo and the future Kayo's struggle to reclaim the title of Shinnosuke's future wife begin.

(Source: Shogakukan, translated)