Is there a way to download a map on the Forerruner 255 music watch?

The FR255 supports routes, but not maps. 

I was not sure exactly what you were asking:

You can  create a route from a prior activity. On the app go to the activity and select the three dots at the top right and then save course . You then need to sync the course with your device.

You only get bread crumb navigation on 2XX series. You need the 9xx to have full downloadable maps.

If you what you can create a route in Garmin Express website and upload to your watch so you have the route and then navigation.

I often create my routes on "on the go maps" then download and upload to Garmin Express so it syncs with my devices.

Google the Garmin manual support pages I cannot post a link here