Well, once you've logged in into SimplyGiving.com, click on your username on the top right of the page, when you are at the dashboard, you'd see a menu on you left, click on profile, and every detail about your organization is on that page. You are able to edit everything from your name to your address.
A few clicks away and a little bit of typing can give you page a fresher look! Check out where you can do all this.
1- Click on Profile
2- Fill in the fields and you are ready to go!
P.s. These are just snippets of what you can change. Ultimately, you can change your whole profile.