There are a few ways.

1) Cut the handle where there is the seam, measuring the length of the new handbrake handle. This method cannot be reversed, obviously. Once you cut, you are must use the new handle, or figure something else out....

2) Remove the entire handbrake boot. This will expose all of the metal underneath, and unless you have another boot to replace it, will look bad. You probably will have to end up taking apart the handbrake. (Remove center console, and detach the cable which is secured by the bolt. This will allow the rubber boot to be removeable.)

I doubt by your question, and my experience with this that you can fit the new handle over the old one, and it will probably look cheesy.

I do not have a new boot for mine yet, so I just cut the rubber handle, and put the new one on. When I make another boot, I will replace the rubber one. Hope this helps.