Good morning, @ckraut
If the devices are already placed, select the devices you want to change, then in the properties dialogue box you will be able to adjust the "Elevation" parameter to be 3' 9".
To update your family's default elevation, open the RFA, select Family Types from the "Properties" section on the Create tab of the ribbon. Then change the "Default Elevation" to 3' 9" and the family will automatically be placed at that elevation in any future projects.
If you have any more questions or if there's anything else I can help you with, reply to this post and I would be happy to continue assisting you. If that is all, let me know so I can go ahead and close this case.
Christian Riley
Technical Support Specialist