transposing and changing key signature on scanned-in sheet music

Hi! I recently scanned in sheet music for a gig I'll be singing at and I need to change the key of the song for me and my duet partner. I'm trying to transpose it to the key of G but it's just not working. I finally got the it to (kind of) transpose to G by choosing "E" in the transpose dialog box, and I checked the "transpose key signatures"
box but it's not working. The notes are where I want them not, but... the key signature is still reflecting Bb on the top two staffs, while I am able to manually change the bottom staff to one sharp reflecting the key of G. How do I get those top two staffs to change?