I try to install a new Python version (3.8) using conda.
!wget -O mini.sh https://repo.anaconda.com/miniconda/Miniconda3-py38_4.8.2-Linux-x86_64.sh
!chmod +x
Update: Originally answered on 2020-30-29, but this answer is now outdated, see the answers above.
Based on these previous answers*, it seems that Google only supports python 2.7 and python 3.6 right now (as of 2020-03-29). However, if you must use python 3.8 you could connect to a local runtime: https://research.google.com/colaboratory/local-runtimes.html
*Previous answers: