Hi, @VeJay, it is not currently possible to edit an exercise. In this case you will need to make a note of the start time and correct duration and create a manual log. Your manual log will capture your steps, heartrate etc that your Fitbit detected during the same time period.
You also might like to add your vote and comments to this popular feature request to allow editing of activities.
Sense, Charge 5, Inspire 2; iOS and Android
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