i Tried apt install python 3.9 and it reply
Building dependency tree
Reading state information... Done
E: Unable to locate package python3.9
E: Couldn't find any package by glob 'python3.9'
E: Couldn't find any package by regex 'python3.9'
You are getting that error because you first need to update the package list and the prerequisites.
sudo apt update
sudo apt install software-properties-common
Then, add the repo ppa:deadsnakes/ppa to your sources list (where you will download python from)
sudo add-apt-repository ppa:deadsnakes/ppa
Make sure to press enter when prompted.
Lastly, install the version of your choice
sudo apt install python3.9
Make sure to read this:
Disclaimer: there's no guarantee of timely updates in case of security problems or other issues. If you want to use them in a security-or-otherwise-critical environment (say, on a production server), you do so at your own risk.
If you are on Ubuntu 19.04 eoan (or any other version unsupported by the deadsnakes ppa), you will not be able to install using the deadsnakes ppa.
What you can do instead, is get the source from Python's official website, and install it manually, as described here.
To do so, first, install the dependencies required to build the Python package.
sudo apt install build-essential zlib1g-dev \
libncurses5-dev libgdbm-dev libnss3-dev \
libssl-dev libreadline-dev libffi-dev curl software-properties-common
then download the tarball and extract it:
wget https://www.python.org/ftp/python/3.9.0/Python-3.9.0.tar.xz
tar -xf Python-3.9.0.tar.xz
Then cd to the extracted directory:
cd Python-3.9.0
Now configure the script:
./configure
Now, to install Python3.9 alongside your current Python installation (if any), use:
sudo make altinstall
Lastly, you can verify your installation using
python3.9 --version