Simply by uninstalling Lr from the old laptop and installing it on the new one. There is no activation process like that used by Photoshop, and Adobe relies on a simple honour system, trusting you to have Lr on no more than two computers in your ownership.
Thank you for your reply johnhawk666
Unfortunately we are unable to uninstall it from our old laptop as it has died - can't get in to it at all.
As John has said this isn't an issue as there is no de-activation process. Assuming this is standalone LR rather than CC, you will however need your serial number which hopefully you registered with Adobe or have written down somewhere.
Download the latest version from here: