You have two choices:

1. You can just set their Trial to no longer be a Trial on the web site (keeping their existing Trial Product Key).  This is the preferred method.  Remember to set the protection parameters on the web site appropriately (you probably want to remove the expiry date).
   You may wish to automate this conversion and combine it with receipt of payment from your customer.  See the 'Online Payment Systems' chapter in the CopyMinder manual (Help|Contents from the CopyMinder.exe menu bar) for more details.
   
2. You can replace their Trial installation with one activated with a new (non-Trial) Product Key.
   To do this, the old .cm file has to be replaced with a new one.  You can do this in one of 2 ways:
   a.    You can re-run CMInstall without the /K option.
   b.    You can delete the existing .cm file (see http://primary.copyminder.com/kb/?View=entry&EntryID=3307  for instructions on finding the old .cm file) and re-run CMInstall (with or without the /K option). You may wish to also delete the .cm.ini file that exists in the same directory as the .cm file so that it does not display the old (Trial) Product Key when they first run.