How do I slow down a Beat Mapped song - whilst keeping the Tempo Track in sync?

I'm using

Mac OS 10.13.2

Logic Pro X 10.2.3

Hello,

I have an an old song recorded in the 1960s with a real drummer which I'm trying to transcribe the bass line from.

I've created a track, dragged in my audio and then used Beat Mapping to create a Tempo Track.

The song is now perfectly in time with the Tempo Track.

Now, I want to slow down the track (its currently at 141 BPM). I want to slow down the song so I can hear what's being played.

How do I do this, whilst keeping the Tempo Track metronome in sync?

My eventual aim is to play the bass line into Logic from a midi keyboard and then annotate the notes with music score.

Thank you for your help.

G