I need help singing soprano?

I'm auditioning for a choir on tuesday, September 16th, and I need to try out for both the alto and soprano parts. I'm usually alto and I don't have a very high vocal range, but it is required that you sing both parts. I did some research online but it told me it could take around 3months to change your voice! How can I help myself learn to sing soprano well enough to be chosen? (If Helps any, i have to sing the first verse of Sing we and chant it)
I need help singing soprano?
Usually they do this because they find that some people who were Alto's are actually stronger as Sopranos and vise versa. If you cannot sing Soprano then don't worry! As long as you are a strong Alto singer then you'll get in as Alto. Don't hurt your voice trying to reach a note that you just aren't ready to reach yet. Tell them before auditioning that you have a limited range and find Soprano difficult but you will try your best to sing it :)

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