(1) Go to Media (by clicking Media on the menu) and then (2) search for the name of the doc you’re replacing (in the example, I just put in “dress” )  and then (3) click the document that you want to replace (in the example it’s the dress code).  (scroll down)

Then you just replace the document with your updated copy:

Browse for your file, and then just click “upload” and you’re done.  You can leave the two other settings alone.