Followed the instructions here: https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-255-series/335468/i-cant-update-my-garmin-255#pifragment-1279=12
But is it normal that my last firmware version is still 15.19? Location is Germany, neither the Connect app nor Garmin Express offer 16.17 or 16.19 for Forerunner 255. Thanks for some advice.