Delete Installation folder
From Joomla! Documentation
Deleting the installation folder
You will not be able to browse to your site or administrator portal until you delete your installation folder. The reason for this is mainly due to security - if someone can access your installation folder they could potentially overwrite your site by running the installer again.
To delete your installation folder manually, use an FTP client and right click on the folder named "installation" and press Delete. As this removes the entire folder and its contents, it may take a couple of minutes to complete. Some components such as Akeeba Backup offer you the opportunity to execute this process by clicking a link, which effectively deletes this folder using certain commands.