I have downloaded Python 3.8 on my Ubuntu 16.04 and if i write python3.8 it shows it is present, but when I write python --version I am getting my
Everything depends on how you installed the python3.8. Many methods wouldn't update default symlinks for you.
If you do something like:
sudo ln -s /usr/bin/python3.8 /usr/local/bin/python and run python --version afterward it should solve your issue.
If you python3.8 binary is not in /usr/bin/python3.8 update your symlink path accordingly.
Keep in mind that some apps dependent on specific features of the lower python version might not work correctly. With Python3 the probability is low though.