I have two python versions in my machine python2.7 and python3.7.4. How can I upgrade my python3.7.4 to python3.8 and set its as default across all user?
I'm going to be blunt here:
The reason python 2.7 is still installed on your system is almost certainly because various scripts and programs, possibly system-essential ones, still rely on 2.7 and haven't been ported to 3.x yet.
Python 2.7 programs do not work in Python3 without extreme rewriting. Your package manager will almost certainly refuse to let you delete 2.7, and you risk breaking your entire system if you try to force it.
I'm not terribly familiar with Kali specifically (and this is neither the time or place to once more point out that Kali isn't intended to be, and shouldn't be used as, a general-purpose desktop system), but I do know that Debian maintainers are working hard to audit all their software to make sure there'll be no more remaining 2.7-reliant scripts well before the sunset.
Also, to ease your fears a bit, Jan 1st 2020 is when the official Python team will stop supporting and maintaining 2.7; most distributions have their own maintenance teams that generally can and will address security issues as they arise for at least long enough for their own software to no longer be dependent on 2.7.