Not to second guess Keyur, but I think one important correction needs to be made to the steps listed.
This is probably going to sound clear as mud, but its really alot easier then this is probably going to read:
When you install a new software image to the WLC, that image becomes the Primary Image and it also becomes the Default image upon reboot. So the idea is that you pre-image download this primary image, and then you reboot everything.
However, the "reset system in X image swap reset-aps save-config" is telling the WLC and APs to swap their image. So the AP will correctly set the new preimage image as its primary, but since the WLC already had the new image as its default, its going to swap over to the backup image. After a reboot, I believe you'll find yourself with a WLC back on your original image since you told the WLC to swap images.
If you read the instructions in the link keyur provided, there is one important step often overlooked. It tells you to set your backup image as your active/default image. "config boot backup"
When you do this, your WLC will be configured that upon reboot it will stay on the current image (which is now backup after your upgrade).
The "swap" command now tells the APs to swap image and the WLC to swap (back to the new image), and you'll find that everything will be done correctly this way.
There is very much a possibility that I don't know what I'm talking about, but I'm pretty sure the commands listed are assuming you have set your controller to use the backup image by default after you install the new image....
