Top right corner of the page, click your account name, which pops up a menu where you click My Contributions. This brings you to a new page showing your contributions to indiegogo campaigns. In line with your Indivisible contribution there's a list of links you can click, one of them is "Upgrade to Higher Priced Perk". Despite the name, I'm positive you're free to make any combination of giving more and choosing a different perk.
This all assumes you have an indiegogo account. If that's not the case, you may have to refund your current pledge and make a new one (with an account, probably would be better)