When the rulers are visible in the WYSIWYG window (after either selecting 'Show Ruler' in the 'View' menu or pressing Ctrl+L), right-click on either ruler then select the units to use:
Changing the ruler units also changes the units of the page size indication that is shown in the status bar at the bottom of the GUI.
Selecting 'Reset' from the pop-up menu resets the rulers to their default positions at the edges of the WYSIWYG window. Selecting 'Hide'
hides the rulers.