Go to the fitbit.com dashboard (click Dashboard near top right of this page. Then 'Log', 'Activities'. Scroll down to 'Activity History', find that workout and click the pencil next to the name to edit it. Can also be done on phone app by clicking exercise tile, click on the workout, then click on 3 dots at top right to go into editing.
I assume you used SmartTrack auto-recognize for recording that. You can avoid this in the future by using MultiSport mode, using side button to select activity and start and stop workout as described here. I find that more reliable and you can be sure you get credit for the full time of your workout.
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