How reset your Apple watch if previous still has iCloud on it ?

You need to contact the previous owner and have them follow these steps:

1. On a computer, go to iCloud.com and sign in with your Apple ID. 
2. Go to Find My iPhone.
3. Select All Devices, then click your Apple Watch.
4. Click Erase Apple Watch. Select Next until the device is erased.
5. Click the remove button  next to your Apple Watch.