How to Install Python 3.9 on Debian 9

STEP 1. Update.

sudo apt update && sudo apt upgrade

SETP 2. Install below packages on your system, which is required for the installation of Python 3.9 from source code.

sudo apt install wget build-essential libreadline-gplv2-dev libncursesw5-dev \
     libssl-dev libsqlite3-dev tk-dev libgdbm-dev libc6-dev libbz2-dev libffi-dev zlib1g-dev  

SETP 3. Download the Python 3.9 source code archive from its official website.

wget https://www.python.org/ftp/python/3.9.7/Python-3.9.7.tgz

SETP 4. Install

tar xzf Python-3.9.7.tgz 
cd Python-3.9.7
./configure
make & make install

SETP 5. Change python3 command

rm /usr/bin/python3
ln -s /usr/local/bin/python3.9 /usr/bin/python3