Yum In RHEL 5

How do I use yum command under RHEL 5 to install and update packages?

The yum command can be run by hand from the shell prompt, called through one of the provided front-end tools, or configured to run automatically at specified intervals.

Update All Installed Packages

The following command displays a list of packages that need to be updated:
# yum check-update
And to install these updates, enter:
# yum update

Package Listing

To ist all available packages, run:
# yum list available # yum list available "package-name"
To list all installed packages, run:
# yum list installed

Package Installation

To install the specified RPM or packages via RHN, enter:
# yum install packageName # yum install httpd

Package Deletion

To remove the specified RPM(s) and it's dependents, enter:
# yum remove packageName # yum remove httpd

Recommend readings:

RHEL yum commands list : Update / Install Packages under Fedora / Redhat Enterprise / CentOS Linux Version 5.x.
CentOS / RHEL Configure Yum Automatic Update Retrieval and Installation
RHEL / Red hat / CentOS / Fedora Send Yum Package Update Notification Via Email