First you’ll need to install the remi repo

sudo wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
sudo wget http://rpms.remirepo.net/enterprise/remi-release-7.rpm
rpm -Uvh remi-release-7.rpm epel-release-latest-7.noarch.rpm

You’ll need to install yum-utils, disable all old php versions, and enable the remi-php74

sudo yum install -y yum-utils
sudo yum-config-manager --disable remi-php56
sudo yum-config-manager --disable remi-php71
sudo yum-config-manager --disable remi-php72
sudo yum-config-manager --disable remi-php73
sudo yum-config-manager --enable remi-php74

If you run into issues you may want to see if you have the webtatic repo installed. I had to disable it to get php to update.

vi /etc/yum.repos.d/webtatic

[webtatic]
name=Webtatic Repository EL7 - $basearch
baseurl=https://repo.webtatic.com/yum/el7/$basearch/
mirrorlist=https://mirror.webtatic.com/yum/el7/$basearch/mirrorlist
failovermethod=priority
enabled=0   <- Disabled by changing to a 0
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-webtatic-el7

And running the yum update should update it to php 7.4

sudo yum update

More information available here.