pip is not updated in the virtual environment

Created a virtual environment in python. I try to update pip with the command

pip install --upgrade pip

But pip is not updated because pip is already installed. Although the pip 9 version is installed in the virtual environment. And it doesn't want to update to the latest version. What could this be about?
Ubuntu 18.04, Python 3.6

Author: Soft, 2020-12-12