Firmamento is a poetic word for "the heavens" or "the sky". It's a much more literary and dramatic term than the common word cielo.
In this passionate song, Luis Fonsi makes a grand, romantic promise: "Te bajo el firmamento si es que así vas a creer" (I'll bring down the heavens for you if that's what it takes for you to believe). He uses this beautiful word to show the incredible lengths he would go to win back his love, making it a perfect example of romantic hyperbole.
Puerto Rican hitmaker Luis Fonsi teams up with Colombian artist Feid for "CAMBIARÉ", a high-energy pop track about being caught in a major mess-up and begging for one more chance. The whole song starts like a desperate message passed through a mutual friend. He begs, "Amigo, si la ves / Dile que tú me viste diferente" ("Friend, if you see her / Tell her you saw me looking different"), admitting that he is pretending to smile while falling apart inside ("Que estoy jugando a ser feliz / Pero por dentro no se siente" ["That I'm playing at being happy / But inside it doesn't feel that way"]). He owns up directly to blowing it: "Que le he fallado completamente" ("That I have failed her completely").
From there, the track turns into an all-out plea for forgiveness. He promises the impossible with lines like "Te bajo el firmamento si es que así vas a creer" ("I'll bring down the sky for you if that's what it takes for you to believe me"), offering endless promises to transform his behavior ("Yo cambiaré" ["I will change"]). You will also hear casual everyday spoken Spanish when he sings "pa' darte to' lo que tú quieras" (short for para darte todo, meaning "to give you everything you want"). It is dramatic, catchy, and totally honest about the panic of losing someone you love.