I thought I can upgrade the python to 3.6 and only use 3.6. So I can remove 2.7.
So I did :
#sudo apt-get remove python2.7
And type Y for confirmation.
The next thing happen is, I lost gimp, inkscape and many other programs. Hahahaha
So, my backbox 5 is depend in python2.7 for its backend. OK, I got it.