Now I have server which installed CentOS, and I want to install Jupyter hub which can only be installed on Ubuntu, is there a way to install Ubuntu on top of CentOS, not side by side?
You're misinformed about JupyterHub's installation requirements. According to their docs, the only requirements are Linux and python v3.5 or newer. Here are the installation instructions:
python3 -m pip install jupyterhub npm install -g configurable-http-proxy python3 -m pip install notebook # needed if running the notebook servers locally
conda install -c conda-forge jupyterhub # installs jupyterhub and proxy conda install notebook # needed if running the notebook servers locally