How to select a voice when two voices are in unison

I am editing a piano piece, I have a passage of 16th notes and some of the high notes are to be held, as a cantus firmus, so I copy the notes to be held and “paste special” them into up-stem voice 1. What I can’t do is to select the “up-stem voice 1” so that I can change it to a half note.

Click the stem and the correct notehead will be selected.