how do I change the version of Python that emacs uses in the python-mode to the latest version that I just installed ?
I tried setting the PATH in my init.el file to the path where the latest version of python resides but its not working.
Set the variable python-python-command. This can be done via customize:

M-x customize-option RET python-python-command RET
Change the value to point to the appropriate binary.
Emacs on Mac for Python - python-mode keeps using the default Python version