How to update/upgrade PHP 7.2 to latest version safely?

Open the terminal and type:

sudo add-apt-repository ppa:ondrej/php
sudo apt update
sudo apt install php7.3 # The latest version in this repo for 18.04 is currently php7.4.  

php7.3 will be updated automatically when updates become available. Alternatively you can build php7.3 from source in Ubuntu 18.04, but it will not be updated automatically, and it also takes more than an hour to build if you want to test it too.

After installing php7.3 you can show php7.3 modules that can also be installed by running the following command:

apt-cache search php7.3 | grep php7.3  

A quick one-line command to install the same packages on php7.3 as php7.2:

sudo apt install $(apt list --installed | grep php7.2- | cut -d'/' -f1 | sed -e 's/7.2/7.3/g') 

From @karel's answer I replace the first command from sudo add-apt-repository ppa:ondrej/php to sudo apt-add-repository ppa:ondrej/php and it works for me. So finally I run:

sudo apt-add-repository ppa:ondrej/php    
sudo apt-get update
sudo apt-get install php7.3