(1) If multiple versions of python are installed in windows, the new version of the pip installation library is installed in the new version.
(2) Solution: Find the path of python.exe. For example, in my computer, python3.5.4 and python3.7.3 are installed.
(3) Find the python.exe path C: \ Users \ lvnianzu \ AppData \ Local \ Programs \ Python \ Python37 of python3.7.3,
(4) Rename python.exe to python3.7.3 then install the library next time (to install opencv as an example) the command is
python3.7.3 -m pip install opencv-python
