I administer an existing site, and accidentally changed some of the formatting on our homepage. In the past, I have been able to press "back" on my browser to undo changes, but for some reason it's not working this time. Does anyone know of a way I can "undo" changes I made to the website under the admin panel?
Your ability to undo changes depends on what you changed and how your site was set up. Some components of Drupal can be reverted, like if you have revisions enabled on your nodes. A more advanced set-up may have "Features" which allow you to revert all sorts of things. It really depends.
For most changes you make in the admin panel, you need to find the setting you adjusted and change it back to its original value. It would help if you described your issue in more detail, like what settings were being adjusted and what version of Drupal you were using.