(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.