To change from miles to kilometers or the opposite, log in to Final Surge and at the upper right under your name is the word "Settings": https://log.finalsurge.com/UserProfile.cshtml
Select Settings and look on the list of User Settings, you'll see "Measurement Units". On the upper right of the User Settings list you'll see a button "Edit Settings". Select Edit Settings and change from "Imperial" to "Metric" or Metric to Imperial.
Changing this setting will not change the units for past workouts that have been imported into your training log from platforms like Garmin, Strava, Polar, etc. Workouts import in whatever unit you currently have your Measurement Units set to. If you change this setting, all future workouts will import using your new Measurement Unit settings. Old workouts will remain as-is.