Forms :: Set Subform To Update A Single Field

So I'm making a database for an office and I'm having a little trouble getting the information linked together. This is my first access project.

Here's the information:

Desktops_TBL
Desktop ID (PK)
Location (FK)
Serial Number
Product Number
Operating System
etc

[Code]....

How I want it to work:

-New users or equipment is entered via a new entry form and stored in the respective table with a dummy Location (i.e. "Backstock"). This part I understand how to do
-To assign a user and equipment to a location, I want there to be a master form for that location. In that master form, there are subforms displaying the current pieces of equipment attributed (if there are any) to that location. A combo box displays the possible choices, and choosing a choice attributes the equipment to that location.

I have the form with subforms set up. The number of records corresponds to the number of locations in the office. Going through the records displays the correct information of attributed equipment in the subforms.

Where I am stuck: What I want is for the user of the database to be able to click on the Desktop serial number combo box, choose a different record, and have it assign the Location from the main form to the Desktop Location FK so that they link. When I try to do this, it writes a new blank record in Desttop_TBL, assigning the Desktop ID to the PC Serial Number spot, but that's it.