You can set the units for a new document if you choose Preferences > Units and Increments with no documents open.
But for the documents you have already created, right-click on the ruler to change the units and save the file. It will retain the units in the file.
Yes I know how to set them they way I want. I just want it to always be set to inches, (permanently).
If you change your settings with no document open it will set your preferences for any new documents you create.
Note, however, that many settings are saved with the document, so if you open an existing file that uses a different measurement your preference will not apply.
This is particularly annoying as there is no way (that I know of) to override when opening someone else's documents. Units isn't a big deal, but for example I HATE that my keyboard increment setting changes when I open other people's documents. There is no good reason for that to be a document-specific setting; I don't care how the document creator preferred to nudge objects, but I know what setting I want to use when editing!
You can set it permanently to inches only for new documents. You can't force a document which has a different unit to open in inches. You can only open each one and re-save it.