That would be a yum upgrade , not yum update. It should work but you need to let yum know what version you want to get rpms from. I guess there are fancier ways of doing this but, I just edit each repo to replace all $releasever with the version number.
name=Fedora Core $releasever - $basearch - Base
name=Fedora Core 4.91 - $basearch - Base
Then, I do a yum upgrade
After that is done, I do a yum update and make sure the /etc/fedora-release shows the new version.