Really, I don't know if I should be writting this to be honest.
WARNING: I shall not be considered responsible if anything breaks.
Fetch the current kernel from here, http://nycdn.netbsd.org/pub/NetBSD-daily/HEAD/latest/amd64/binary/kernel/
Assuming you have it in your Downloads directory, verify the checksum and unpack it.
cksum -a MD5 netbsd-GENERIC.gz
Now, in the same directory as root/su
mv netbsd-GENERIC /
chmod 755 netbsd-GENERIC
chown root netbsd-GENERIC
mv netbsd netbsd_9
mv netbsd-GENERIC netbsd
If you want to revert to the original kernel, again as root/su
mv netbsd netbsd_HEAD
mv netbsd_9 netbsd