[ubuntu-cloud] EC2, "2.6.32-21-server" kernel instead of "2.6.32-305-ec2"?

[Jack Bates]

When I run the EC2 image "ami-fd4aa494", it boots kernel
"2.6.32-305-ec2",

$ uname -r
2.6.32-305-ec2
$ 

However kernel "2.6.32-21-server" is also installed,

$ ls /boot/vmlinuz-*
/boot/vmlinuz-2.6.32-21-server  /boot/vmlinuz-2.6.32-305-ec2
$ 

How do I boot from this other kernel instead?