Java.script: How to update a text label on an input field.

I have a form where the text label needs to change if a user makes a choice.

e.g. Diameter (thou) / Diameter (mm)

BTW, don't flame me on this, I didn't invent the part number system.

How do I find the DOM element related to a given field (or field ID).

I've spent ages looking at the structure in VS, but I can't see how to make the link.

I know a work round might be to hide the text label and use a field instead, but before I do that, is it possible just to change the existing text label.