You can achieve this by applying below simple steps -
- Check python version on terminal - python --version
 - Get root user privileges. On terminal type - sudo su
 - Write down the root user password
 - Execute this command to switch to python 3.6 -
 - update-alternatives --install /usr/bin/python python /usr/bin/python3 1
 - Check python version - python --version
 - Done.