By Clint Brown
A common question that I come across is; "How do I change Inventor's units" the answer is quite straightforward.
With the file in question opened up, you can click on Tools > Document Settings > Units
Make the changes as needed.
This is great if it is a one off file, but if you are looking to change your template file, it is a bit more involved, you could create a new file from your default template, change the units as described above, and then save it back as a template file.
Or if you are using Inventor 2013, there are some nice easy tools for switching your units by default (in the old days Pre 2013 it was a bit of work, see the attached PDF
All you need to do now (2013 onwards), is make a change to your default template on your Welcome screen (as shown below)
A word of caution would be to ensure that you backup your current template files, as this operation could overwrite any customisations that you have done to your templates.
The easiest way to find the location of your current templates files is to have a look at the template location in your project file.