To change the balance of a Fund on the website, log into EveryDollar.com on a desktop or laptop computer and click the Fund line, then click Fund Details on the right side of the screen. That will pull up the Original Balance field.
Just change the number in that field to whatever you like and click Save, and then you should be set.
Quick note - if you change the Original Balance of the Fund in a future month's budget, it will change that number for all the previous months as well.