How to properly update a program in ubuntu?

Question

ubuntu

I'm new to Linux, I installed ubuntu. It already has python 2 version installed, I need 3. I decided to remove the second version by hand. There are many different packages, I look through dpkg -l | grep python. Deleted them and broke everything. The terminal does not work, navigation buttons disappeared from nautilus. I had to reinstall the OS. In Windows, all programs were installed in one folder and they could be easily removed, but in Ubuntu the program is decomposed into different directories and I don’t know which ones. How to update / remove programs correctly?

Answer 1

The second version does not need to be demolished, many components are tied to it.
The third is installed via:

sudo apt update
sudo apt install python3

At the beginning of the .py file we write:
#!/usr/bin/env python3