You can do that on the Pixnub website. Here is a link to the page. This page is also under the support tab of the website.

When you enter the key, it will show both slots.  You can either clear all activations and reactivate everything or just clear the one computer.  If you go into the computer that you currently have activated, then go to a plugin and to Manage License, it will show you the activation slot that computer is using. So the one you need to clear is the other slot.

If that sounds too confusing then just clear all of the activations and reactivate everything again.

Manage Licenses – User