If we could assume you already did the following...
yum clean all
signal-event post-upgrade; signal-event reboot
yum update
then
Try the instructions under "2 Yum Update" here
http://wiki.contribs.org/Updating_to_SME_7.2If you recently did a yum
upgrade...
then
Read the entire page for that link for what may apply to your system.
Also as a suggestion you could search the forum & wiki & bug tracker & google for your particular error i.e.
"Cannot find a valid baseurl for repo"
Which in most cases, could lead you to possible solutions.
HTH