If you have created your site with a free trial from concrete5 this is not possible. If you are using a different hosting provider read below.

You need to use a FTP client such as Core FTP to access the themes folder to make html changes to your page, the files you need to change will be default.php and main.css, it is best to duplicate these files and rename them as defaultbackup.php and mainbackup.css before editing so you can always go back to how it was originally.

Your ftp details will be available from your account with your hosting provider.