The way the system works is that the files that Garmin, Strava, Polar, etc. sends us have to be imported in either Miles or Kilometers. There is not a setting in the file that tells us how the workout was recorded. So, depending on your Imperial/Metric setting, we import the data as that type when it comes into our system. Once imported, the workout will stay in that measurement unit that it was imported in.
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.
However, all totals on your dashboard and paces will display in your new settings, even if the workouts were imported as a different measurement unit.