How do you uninstall the package manager "pip", if installed from source?

pip uninstall pip will work  

That way you haven't installed pip, you installed just the easy_install i.e. setuptools.

First you should remove all the packages you installed with easy_install using (see uninstall):

easy_install -m PackageName

This includes pip if you installed it using easy_install pip.

After this you remove the setuptools following the instructions from here:

If setuptools package is found in your global site-packages directory, you may safely remove the following file/directory:

setuptools-*.egg

If setuptools is installed in some other location such as the user site directory (eg: ~/.local, ~/Library/Python or %APPDATA%), then you may safely remove the following files:

pkg_resources.py
easy_install.py
setuptools/
setuptools-*.egg-info/

If you installed pip like this:

 - sudo apt install python-pip
 - sudo apt install python3-pip

Uninstall them like this:

 - sudo apt remove python-pip
 - sudo apt remove python3-pip

I was using above command but it was not working. This command worked for me:

python -m pip uninstall pip setuptools