You can do it in two ways.
You can get accounts and get the specific child and update and save the child
var ref = new Firebase(fbUrl + '/accounts');
var accounts = $firebase(ref);
var child = accounts.$child(key);
// Set any value to child and save
// child.name = 'changed name';
child.$save();
(Or)
Get the child element directly
var ref = new Firebase(fbUrl + '/accounts/' + key);
$scope.account = $firebase(ref);
// Set any value to account and save
// account.name = 'changed name';
$scope.account.$save();