If you notice, Microsoft Word uses horizontal and vertical rulers to align paragraphs, tables, images etc.By default, the unit for these rulers is set to inches but, you can change it to a unit of your choice easily. Follow these simple steps.

Open Word, Click on the File menu and choose ‘Word Options’ as shown in the screen-shot below.
Once in the Word Options window, switch to ‘Advanced’ tab. In the right-pane scroll until the ‘Display’ section is visible to you. When found, select the desired unit from the from the drop-down list and click on ‘OK’.
That’s it!