If the Now Playing screen is displayed, swipe down to minimize.  Scroll up on the sources at the top of the screen until you see ‘Your Sources’.  The Line-In should be there.

I can see Line In, and it is currently in use for my “living room”, but the “bathroom”, where I use my Roam still shows the radio. That’s what I can’t figure out how to change.

At the top of the screen, select ‘Your System’.

Then select the Roam.

That ‘selects’ the Roam to accept changes, so you can then choose the ‘Line In’ (which I don’t have, so I’m unsure of where it shows up on the main page, but likely on ‘Your Services’. 
 

Conversely, you can always ‘group’ the Roam with any other room that is already playing the turntable.