send data to firebase with flutter

I am trying to send the data to firebase in my project which I was able to , yet I want the data to override the variable. The one I am using is adding at all the time

 setState(() {
      velocity = newVelocity;

      if (velocity > highestVelocity) {
        highestVelocity = velocity;
      }
    });
      future= new Future.delayed(const Duration(seconds: 5));
     _firestore.collection('collectionName').add({
                      'velocity' : velocity

         }); 
}

I think there is update function to override the var. Can I someone help me with that?

Solution 1: Frank van Puffelen

To update an existing document, you must know the ID of that document. Once you do, you can update it with:

_firestore.collection('collectionName').doc("theIdOfYourDocument").set({
  'velocity' : velocity                  
}); 

Also see: how can I set my document id as user uid in cloud fire store?