What makes you think such a kernel package is available in the standard repositories?
Try:
yum --enablerepo=updates-testing list kernel
If that package is not available in the repositories yet, you can visit the Fedora Updates System ticket,
https://admin.fedoraproject.org/upda....6.40.3-0.fc15
and verify the state of the update. At the time of writing this reply, it is still
pending with target
testing, which means it is waiting to be pushed into the updates-testing repository.
Meanwhile you can follow the link to the Fedora Build System and download the package manually from there.