If you are a programmer trying to write code using OpenGL 4.5, you do not need to "download the API" from Microsoft or Khronos. You need the development libraries.
from the manufacturer:
I cannot directly provide downloads or files, but here are the official ways to get OpenGL 4.5 support: opengl 4.5 download
Most modern C++ developers use a library loader generator: If you are a programmer trying to write code using OpenGL 4