Layers text vs rectangle

Hello, I'm trying to change the lyrics in a vocal score for a musical production. Most times, Acrobat pro recognizes the text and it's quite easy to change it. Sometimes, it doesn't work. If I just create a rectagle and put it over the scanned text and try to write new text over it, the white rectangle stays in the front layer and my text just vanishes behind it - impossible to read. Is there a way to put the rectangle in the back and the text in the front layer? I can't find it...