Removing an Anti-Virus license from a machine that no longer works

Of the 3 PCs that my subscription covers, I only have 2 of them (the third has died - so I can't de-register this machine), and now that I have bought a replacement I'm not able to use the license of the dead machine.

 

How can I de-register a dead machine to make the license available for a new machine?

 

Kind regards,

Kim...