RHN/Yum update from Red Hat Enterprise Linux 5.0 to 5.1

No. 5.1 through 5.5 have come and gone, and only 5.6 remains.

As Ignacio said, there's no good way to freeze your install at a specific subversion. In order to have a properly updated system you will have to migrate up to 5.6. However, along the way glibc has been updated, which can result in some issues during the update process. The safest way to update your system will be to run these commands in this order:

1. yum clean all
2. yum update yum
3. yum update glibc
4. yum update

Then grab some coffee, a copy of Moby Dick, and come back the following day.

You can update to a specific release by using the DVD. Mount the 5.1 DVD as a local repo[0] and update with:

yum update --noplugins

[0] https://access.redhat.com/kb/docs/DOC-9744