RHN/Yum update from RHEL 5.0 to 5.1
 

I need to update a server from RHEL 5.0 to 5.1

When I use RHN/Yum and do a yum update the RHEL 5.0 server gets updated to 5.6

Is there a way to configure RHN/yum to update the server only to RHEL 5.1 ?

In RHN I only see a general "Red Hat Enterprise Linux (v. 5 for 32bit x86)" software channel and not specific for the update versions like 5.1

You need to create your own custom channel for this. Then move all the errata upto the release of 5.2 into that channel, then you can upgrade just with those errata. I'm not sure you can do it in RHN, its probably only possible in Red Hat Satellite.