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 Extend concrete5 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 you'll see a new screen where you have to confirm the action. If you enable the checkbox Yes, remove the package's directory from the installation directory, concrete5 will remove the directory and files of the package as well. Most add-ons are pretty small; it usually doesn't hurt to keep the package files.
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 uninstalled it. The process to install a theme works the same way; click on themes at the bottom in the dashboard and you'll get a list of available themes.