microposts_controller.rb

  # PATCH/PUT /microposts/1
  # PATCH/PUT /microposts/1.json
  def update
    respond_to do |format|
      if @micropost.update(micropost_params)
        format.html { redirect_to @micropost, notice: 'Micropost was successfully updated.' }
        format.json { render :show, status: :ok, location: @micropost }
      else
        format.html { render :edit }
        format.json { render json: @micropost.errors, status: :unprocessable_entity }
      end
    end
  end

microposts_controller_test.rb

  test "should update micropost" do
    patch micropost_url(@micropost), params: { micropost: { content: @micropost.content, user_id: @micropost.user_id } }
    assert_redirected_to micropost_url(@micropost)
  end