With the measurement tool active, right-click anywhere on the document. This will bring up a menu. Here you can now select "Change Scale Ratio" to set a new ratio.
Hey Karl, is there any way to keep the settings once changed as they always defer back to 1" = 1"
No, not with Adobe Acrobat. The scaling settings are stored in the PDF file, and they are usually created by the application that creates the PDF file in question. Most PDF files don't need a scaling setting, because nobody will ever measure anything in them. It's mostly CAD files and maps that benefit from this information, so your copy of AutoCAD or BlueBeam or whatever else is used to create such a file should add that information.
There are ways to get a scaling into a PDF file (or more accurately, associate it with a certain portion of a page) by using custom software. If this is something you are interested in, please feel free to get in touch with me directly. You can find my contact information on my profile page (click on my name in blue just above this comment).