All the notes on a five-line staff are slash notation with stems (that play), but as this is a chords part with a steady rhythm, I want to change the notes to rhythmic slashes that don’t play. I haven’t found the place to make the change. Where…?
You can suppress playback of notes by selecting Supress playback in Properties.
Or, I’ve just thought maybe you mean you want non-rhythmic slashes eg the four-slash-per-bar-with-no-stems slashes. In this case, select the region that you want it to appear, open the repeat popover and enter slash.