Hi - in 'Classic' mode Connect, click on the little down-arrow to the right of your user name towards the top right hand corner of the Connect page, and in the drop-down click on 'Settings'. In the Settings page, select 'Display Preferences', and set Measurement Units to Metric.Which is fine if you have a Garmin Connect login.
In 'Modern' mode, I think it's still under Settings / Display Preferences, but I can't remember how you get there, and I don't want to switch modes in case I can't switch back again :)