You can use following commands to upgrade php from 7.2 to 7.4 on Centos 8:

dnf install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm

dnf install https://rpms.remirepo.net/enterprise/remi-release-8.rpm

dnf module list php

dnf module reset php

dnf module enable php:remi-7.4

dnf install php php-cli php-common