Can I update to python3.7 from python2.7 in my macOS 10.14.1? I have macOS 10.14.1. I installed Python3.7 but can see that python2.7 already installed in my os. Can I remove 2.7 and just keep 3.7? Will it affect other apps?
You don't need to remove one in order to install the other. You can have both python 2.7 and python 3 on your Mac. (In fact, python 2.7 is part of the OS and can't really be removed).

You just need to make sure that you invoke the correct one, either by typing python3 on the command line, or using #!/usr/bin/env python3 in scripts.