What Are Some Ways to Make My Voice Higher (naturally)?

I want to write lyrics to Zelda's Lullaby and sing it next years talent show (it's in April) but I can't sing those high notes. I would say I have the one of the highest voice in school (in choir, I'm in Soprano). I wanna learn how to sing the high notes but I can't sing it! What are some ways I can get my voice higher naturally in over a year? And also get it healthier. Please, I love the song and have been humming the song sice I was 5 (and I'm 13 now). Please!