Make sure to upgrade Ubuntu’s repositories:

sudo add-apt-repository ppa:ondrej/php # Press enter to confirm.
sudo apt update

Install 7.3 core

apt install php7.3-bcmath php7.3-bz2 php7.3-curl php7.3-gd php7.3-intl php7.3-json php7.3-mbstring php7.3-readline php7.3-xml php7.3-zip

Install 7.3 for website

apt install php7.3-fpm

apt install libapache2-mod-php7.3

Copy and paste, then run php -v to confirm PHP 7.3 (CLI) is installed.

Remote the previous PHP version

apt purge php7.2 php7.2-common # Change 7.2 with whatever current version you have.