add how to update whl in build_from_source.md

Author: luotao1

doc/getstarted/build_and_install/build_from_source_cn.rst

@@ -32,6 +32,11 @@ 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：
+
+.. code-block:: bash
+
+   pip install build/python/dist/*.whl -U
 
 .. _run_test:
 

doc/getstarted/build_and_install/build_from_source_en.rst

@@ -36,6 +36,11 @@ 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.
+
+.. code-block:: bash
+
+   pip install build/python/dist/*.whl -U
 
 .. _run_test: