Here are steps I took to upgrade from PHP 7.0 to 7.4 on Ubuntu 18.04 LTS.

Login as root, otherwise use sudo for all commands.

# use 3rd party ppa repo
add-apt-repository ppa:ondrej/php
apt -y update
apt -y upgrade
# install php-core
apt install php7.4 php7.4-common php7.4-cli
# add all the php extenstions your app needs
apt install php7.4-curl php7.4-json php7.4-gd php7.4-mbstring
apt install php7.4-intl php7.4-bcmath php7.4-bz2 php7.4-xml
apt install php7.4-readline php7.4-zip php7.4-mysql
# I'm using apache prefork MPM, so install it
apt install libapache2-mod-php7.4
a2enmod php7.4
# check version. Success!
php -v
# for good measure, reboot apache
systemctl restart apache2.service
# Finally, purge the old PHP versions
apt purge php7.0 libapache2-mod-php7.0

Finally, run update on more time to get all the latest updates.

apt -y update
apt -y upgrade
apt -y autoremove
apt-get --with-new-pkgs upgrade