Update related tables laravel

I have been looking at the documentation for a while and I think it is correct, however it does not work as it should, instead of updating me, it creates a record.

The theme is as follows. I have a table with sellers and others with products, the idea is to update a product from a certain vendor.

These are the data.

The route

Route::resource('vendedor.producto', 'vendedor\vendedorProductoController',['except' => ['create','show','edit']]);

and the structure of the route when we put it in the browser becomes like this.

vendedor/{vendedor}/producto/{producto}

In my case, for example, it would be something like that

vendedor/95/producto/100

which would be that of seller 95 update the product 100

the seller model

class vendedor extends User
{


    protected $table = 'vendedores';



    public function productos()
    {
        return $this->hasMany('App\modelos\producto');
    }
}

the products model

    class producto extends Model
    {
       use SoftDeletes;

       protected $table = 'productos';

       protected $dates = ['deleted_at'];

       protected $hidden = ['pivot','deleted_at'];

       const PRODUCTO_DISPONIBLE = 'disponible';
       const PRODUCTO_NODISPONIBLE = 'no_disponible';

       protected $fillable = ['nombre','descripcion','cantidad','status','imagen','vendedor_id'];

       public function estaDisponible()
       {
         return $this->status == product::PRODUCTO_DISPONIBLE;
       }

       public function categorias()
       {
          return $this->belongsToMany('App\modelos\categoria');
        }

       public function vendedor()
       {
          return $this->belongsTo('App\modelos\vendedor');
       }

       public function transacciones()
       {
          return $this->hasMany('App\modelos\transaccion');
       }

}

The controller with the update method

public function update(Request $request, $idvendedor)
    {
        $vendedor = vendedor::findorfail($idvendedor);


        $datos = [
            'nombre' => $request->nombre,
            'descripcion' => $request->descripcion,
            'cantidad' => $request->cantidad,
            'imagen' => '1.jpg',
           'vendedor_id' => $vendedor->id,

                ];
        $producto =new producto($datos);


        $vendedor->productos()->save($producto);



    }

Well, do not update it, simply create a new record as if it were the store method

---------------- EDITION1 ------------------------------ --------------------------

I have made some modifications to the code and it still does not work.

I put the modifications

public function update(Request $request, $idvendedor,$idproducto)
    {
        $vendedor = vendedor::findorfail($idvendedor);
        $producto = producto::findorfail($idproducto);


        //$producto = new producto();

        $producto->nombre = $request->nombre;
        $producto->descripcion = $request->descripcion;
        $producto->cantidad = $request->cantidad;
        $producto->imagen ='9999.jpg';
        $producto->vendedor_id = $vendedor->id;


        $vendedor->productos()->save();


    }

In theory it has to work with this line

$producto = producto::findorfail($idproducto);

instead of this

 //$producto = new producto()

but I have commented it simply so that you see that I have used both options and it does not work for me, the error that gives me is this

Argument 1 passed to Illuminate\Database\Eloquent\Relations\HasOneOrMany::save() must be an instance of Illuminate\Database\Eloquent\Model

And here is the migration in case it clarifies something

PRODUCT MIGRATION

 public function up()
    {
        Schema::create('productos', function (Blueprint $table) {
            $table->increments('id');
            $table->string('nombre');
            $table->string('descripcion');
            $table->integer('cantidad')->unsigned();
          $table->string('imagen');
            $table->integer('vendedor_id')->unsigned();
            $table->timestamps();
            $table->softDeletes();
            $table->foreign('vendedor_id')->references('id')->on('users')->onDelete('cascade')->onUpdate('cascade');
        });