You don't specify if it is really the wrong time, or if you want it to read 24 hour military notation instead.
If you are not sure, since you just set it up, maybe wait another 9+ hours to see what the time reads then.
If the issue is that you want military time,
How do I change to a 12 or 24 hour clock?
- From the fitbit.com dashboard, click the gear icon
> Settings > Personal Info. - Under Advanced Settings, find Clock Display Time and select whether you want the time shown in 12-hour or 24-hour clock units.
- Click Submit and sync your device.
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.