how to update python in raspberry pi

First update the Raspbian.

sudo apt-get update    

Then install the prerequisites that will make any further installation of Python and/or packages much smoother.

sudo apt-get install -y build-essential tk-dev libncurses5-dev libncursesw5-dev libreadline6-dev libdb5.3-dev libgdbm-dev libsqlite3-dev libssl-dev libbz2-dev libexpat1-dev liblzma-dev zlib1g-dev libffi-dev

And then install Python, maybe by downloading a compressed file?

example 1 :

wget https://www.python.org/ftp/python/3.8.0/Python-3.8.0.tgz    

Extract the folder :

sudo tar zxf Python-3.8.0.tgz

Move into the folder :

cd Python-3.8.0

Initial configuration :

sudo ./configure --enable-optimizations

Run the makefile inside the folder with the mentioned parameters :

sudo make -j 4

Run again the makefile this time installing directly the package :

sudo make altinstall

Maybe You already did it but You don't know how to setup the new version as a default version of the system?

Check first that it has been installed :

python3.8 -V

Send a strong command to .bashrc telling him who (which version) is in charge of Python

echo "alias python=/usr/local/bin/python3.8" >> ~/.bashrc

Again! Tell him because .bashrc has to understand! I am joking - You have to source the file so the changes can be applied immediately :

source ~/.bashrc

And then check that Your system changed the default version of Python to Python 3.8

python -V

The failure depends on many factors : what dependencies are installed, what are the packages added to the source_list.d, some inconvenient coming up during the installation. All may give you more information than you think, just read carefully. Hope it helped.

Follow below commands to install the version which you want:

tar xf Python-3.x.x.tar.xz
cd Python-3.x.x
./configure --enable-optimizations
make
sudo make install

once completed run python -V

To all of you who got problem with freezing RPi 3 in step:

sudo make -j 4

just change it to:

sudo make -j 2

or simply:

sudo make 

Best regards