Changing assigned teacher for letter of rec

<p>My son was filling out the school forms section for one school (Northeastern), and had planned on assigning both teachers to send the recs. He clicked on one teacher, and now there is no opportunity to click on the other (i’m guessing since the school only requires one letter). There doesn’t seem to be any way to unclick the first teacher. Does anyone know how to do this? Also, does the teacher rec have to be from a core course, or could it be from Computer Science? The common app has NOT been submitted yet
Thanks</p>