WordPress says you have to upgrade your insecure version of PHP 7.0. PHP 7.4 is the current version.
You have to add the repository, then update.
Ubuntu:
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
Debian:
sudo apt install apt-transport-https lsb-release ca-certificates
sudo wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
sudo sh -c ‘echo “deb https://packages.sury.org/php/ $(lsb_release -sc) main” > /etc/apt/sources.list.d/php.list’
sudo apt update
Change to the php version.
sudo update-alternatives –config php
Enable the correct version in apache.
sudo a2enmod php7.4
Disable the old version.
sudo a2dismod php7.0
Then restart apache2.
sudo systemctl restart apache2