You don't have to change it. I didn't look at your link until now.
Just connect the 5V from the buck converter DIRECTLY to the 5V pin of the arduino with NOTHING plugged into the barrel jack
and with NO USB connected. This will work. You cannot use the barrel jack with that buck converter.
If you had bought this one , you could adjust it:
http://www.miniinthebox.com/lm2596s-power-module-lm2596-dc-to-dc-buck-converter_p1113808.html?currency=USD&litb_from=paid_adwords_shopping&gclid=CjgKEAjwzIucBRDzjIz9qMOB3TASJABBIwL11WdjYbLCV25sDoT4wpcoHVyCzutB9oH-vop-aqFoavD_BwE