Install the Latest PostgreSQL Yum Repository for CentOS
Keeping PostgreSQL updated via yum should be done via the pgdg yum repository supplied by pgsqlrpms.org.
The latest CentOS PostgreSQL repo can be viewed here.
You will need to download the repo and then install using rpm as shown below.
wget http://yum.pgsqlrpms.org/reporpms/8.4/pgdg-centos-8.4-1.noarch.rpm rpm -Uhv pgdg-centos-8.4-1.noarch.rpm
Another helpful postgresql install and management link is located here.