You need to buy a new set of copper/steel rivets from Brooks, drill all of the rivets out of the current top, install them, and then use an anvil to hold the rivet head while you use a peening hammer to lock the rivet in place from the rear. If you replaced them with copper heads, you'll need to carefully flatten the rivets on the top of the saddle, taking care not to damage the top as you flatten them.
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There is a sequence you need to follow that I forgot about. Attach the nosepiece first, thread the tension nut all the way down the bolt, insert the bolt through the nose, and put that assembly on the rails. The cover should be loose enough that you can slip the rear rivets in place and peen them over. Tension the saddle to your desired feel.
*/edit*
If you are even a little bit concerned that you might be in over your head, you or the shop can send the saddle back to Brooks, who can repair it for you. Particularly old shop hands may have some experience fixing leather saddles, and if all else fails, you can try to find a leather worker (such as that at a horse tack shop) that is willing to work with you.
The only way to preserve your warranty would be to have Brooks or one of their authorized people repair the saddle.
I would also add: it's going to be expensive to pay someone else to do it, the process takes at least 30-45 minutes for someone who has done it or similar work before.