If you're talking about Lightroom and not about Photoshop, you don't see one because there is no deactivation for Lightroom. Just uninstall it from the old and re-install it on the new.
What Hal said.
As with all Adobe software, Lightroom permits you to install and run the software on a maximum of two computers at any one time.
Unlike other Adobe software, there is no activation mechanism (yet) built in to Lightroom (so no Help > Deactivate command) so the two activation limit is not policed by the software. It's up to you to abide by it.