Currently, there is no easy way directly in Garmin Connect to convert a multisport activity to a normal one. Although you can export the individual legs independently, they import back as a multisport activity again.
Appart of editing the FIT file with an external tool, there is a relatively easy solution, though, anyway - the legs of multisport activities are being synced to Strava as individual activities, so it is enough to export them as original (FIT file) from Strava, and then import them back to Garmin Connect (do not forget to back up & delete the multisport activity in GC first).
If you do not use Strava yet, just create a free account, and export the multisport activity to Strava manually.
