Hi Ben,
I see you are relatively new to the forums... welcome!
Am I correct when I interpret your above post to mean that you are using Notation Editor to enter a melody on the Melody track and that the sound you have set the Melody track to (which is MIDI, by the way), is a violin?
Then, when you get to the chorus, you want that violin sound to change to a guitar sound? (This is called a 'Patch Change' in MIDI jargon.)
If that's the case, then read on

To do the above...
1. on the Chordsheet, click on the bar that you want the change to occur at
2. right-click and select "Bar Settings" (the shortcut is F5 if you run BIAB Windows)
3. on the row of controls for the 'Melody' track (#1 on the image below), click on the 'Patch Change' button and then select the sound you want (#2).
Regards,
Noel