Assuming you have the Active Directory plug-in installed and have the appropriate permissions, the following code will allow you to set the new description of the computer object.
Inputs: computerAD (AD:ComputerAD), newDescription (string)
System.log("Old Description: "+computerAD.getAttribute("description"));
System.log("Setting new description...");
computerAD.setAttribute("description",newDescription);
The above code, when placed into a workflow with the inputs bound, will prompt you for a computerAD object and Description. Upon submitting, the code will display the current description, then proceed to set the new description as defined.
This code addresses the computer object in AD, I'm unsure what you mean by "Local"