Simple solution is edit .bashrc and put this line:

alias python3=python3.7
Whenever you will write python3 it will replace it with python3.7.

Or you can use command update-alternatives which is preferred i.e:

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 2
So here python3.7 will have higher priority then python3.6. Then use:

sudo update-alternatives --config python3
Press the enter key if you are satisfied