If you look at an NPC and then enter the command editnpc in the console (confirm with enter without further options), this window will appear first:
With the shirt symbol you can adjust the clothing and with the sword symbol a weapon or whatever else you can hold in your hand.
Once you have clicked on the clothing, double-click on your selection and you will be taken back to the main window;
The same applies to equipping weapons or items (double-click on the selection).
Only when you're finished, click "Apply" in the bottom left.