Changing machine's name

["Paul Smith" <phhs80@xxxxxxxxx>]

Dear All

After today's updates, the command line is

[psmith@unknown-00-14-85-38-2c-93 ~]$

and before it was

[psmith@mypc ~]$

How can I replace unknown-00-14-85-38-2c-93 with mypc?

Thanks in advance,

Paul