Here is how I installed pyenv-win:
1 - Uninstall Python (your current version when you type the command python -V)
2 - Install pyenv-win and add it to the PATH (here are some optional steps to install it via chocolatey, which adds it directly to the PATH)
2.2 - Install chocolatey
2.3 - In an administrator powershell run: choco install pyenv-win