How do I update a single item in an ObservableCollection class?

You can’t generally change a collection that you’re iterating through (with foreach). The way around this is to not be iterating through it when you change it, of course. (x.Id == myId and the LINQ FirstOrDefault are placeholders for your criteria/search, the important part is that you’ve got the object and/or index of the object)

for (int i = 0; i < theCollection.Count; i++) {
  if (theCollection[i].Id == myId)
    theCollection[i] = newObject;
}

Or

var found = theCollection.FirstOrDefault(x=>x.Id == myId);
int i = theCollection.IndexOf(found);
theCollection[i] = newObject;

Or

var found = theCollection.FirstOrDefault(x=>x.Id == myId);
theCollection.Remove(found);
theCollection.Add(newObject);

Or

var found = theCollection.FirstOrDefault(x=>x.Id == myId);
found.SomeProperty = newValue;

If the last example will do, and what you really need to know is how to make things watching your ObservableCollection be aware of the change, you should implement INotifyPropertyChanged on the object’s class and be sure to raise PropertyChanged when the property you’re changing changes (ideally it should be implemented on all public properties if you have the interface, but functionally of course it really only matters for ones you’ll be updating).