What exactly are you trying to do? Simple units change so that you 7.3709 inches is now 187.22mm, or just scale by 1/25.4 so that your 7.3709 in is now 7.3709mm?
For the former, I no longer have Autocad so I can't confirm this but this process seems correct:
Type AECDWGSETUP in the command line
Click the Units tab in the Drawing Setup dialog box
Select the desired units under Drawing Units
Set the Precision to the number of decimal places or fractional size
Set a different angle type if needed
Select a unit to apply automatically to imported content under Insertion scale
AutoCAD drawings don't necessarily have units, from a certain point of view.
You can force it either way by setting a system value (per instructions above from lucky-guesser) or by scaling the drawing with the correct ratio.
The system UNIT setting only works if you mean to use inches and millimeters.
If you're drawing in meters or kilometers or yards or miles then the settings aren't going to help you.
Instead, the preferred strategy is to use 1 and only 1 unit system for all parts on the drawing, and don't mention the units in the dimensions. On the drawing title block, include a note "all dimensions in furlongs" or whatever you happen to be using. Then you can just ignore these settings and cultivate habits that keep you out of this trouble.
When importing drawings that you didn't create, chances are you'll have to deal with units. There are different ways, depending on what you imported.
DIK has it right. You can also enable alternate units and have both imperial and SI displayed.
Type DDIM and edit the dimstyle you are using.
You can also create a new DimStyle with the factors you want and apply that to your dimensions
True, but forcing it with overrides and multiplication factors can invite errors and complications. It's a great way to trip up the noobs that try to work on your drawing later. KISS
Do others have to edit/revise drawings you make Dik?
Does it ever turn into a "learning moment" for the juniors to discover features of AutoCAD they typically don't use, in order to deal with workarounds from yesteryear?