Help with W670 (White) Streamliner

Today Kayla brought home her W670 (White) Streamliner.  She loves her vintage machine but can’t get it out of the zig-zag stitch that she used for a quick applique project.  After putting our heads together and consulting the online instruction manual we still can’t seem to make it work.  Any help or suggestions getting the machine back to a straight stitch?