Carry out the following steps if you want to remove an add-on:
You don't like the add-on you just installed? Go to Add Functionality again and locate the previously installed add-on. Click on the Edit button next to the add-on.
At the bottom, you can find a button Uninstall Package. Click on it and the add-on is no longer listed under the installed add-ons; it's now listed on the right where all installable add-ons can be found.
By connecting your site to the concrete5.org marketplace, you gained direct access to all add-ons uploaded to the marketplace. We've installed an add-on and also removed it. The process to install a theme works the same way; click on Add Functionality but this time locate More Themes and pick a layout.