Merge pull request #7471 from luotao1/build_doc

add how to update whl in build_from_source.md

Author: luotao1

doc/getstarted/build_and_install/build_from_source_cn.rst

@@ -32,6 +32,16 @@ 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，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

@@ -36,6 +36,16 @@ machine or copy it to the target machine.
 
    pip install build/python/dist/*.whl
 
+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: