add another method of update whl

doc/getstarted/build_and_install/build_from_source_cn.rst

@@ -32,10 +32,15 @@ PaddlePaddle主要使用 `CMake <https://cmake.org>`_ 以及GCC, G++作为编译
 
    pip install build/python/dist/*.whl
 
-如果机器中已经安装过PaddlePaddle，请卸载再安装：
+如果机器中已经安装过PaddlePaddle，有两种方法：
 
 .. code-block:: bash
 
+   1. 先卸载之前的版本，再重新安装
+   pip uninstall paddlepaddle
+   pip install build/python/dist/*.whl
+
+   2. 直接升级到更新的版本
    pip install build/python/dist/*.whl -U
 
 .. _run_test:

doc/getstarted/build_and_install/build_from_source_en.rst

@@ -36,10 +36,15 @@ machine or copy it to the target machine.
 
    pip install build/python/dist/*.whl
 
-If the machine has installed PaddlePaddle before, please uninstall it first and reinstall it again.
+If the machine has installed PaddlePaddle before, there are two methods:
 
 .. code-block:: bash
 
+   1. uninstall and reinstall
+   pip uninstall paddlepaddle
+   pip install build/python/dist/*.whl
+
+   2. upgrade directly
    pip install build/python/dist/*.whl -U
 
 .. _run_test: