Attachments are uploaded in the Files page, which is found in the Setup drop down list in the menu bar at the top of the admin site.
In the attachments page simply click on the add new attachment button to select a file to upload. Documents are restricted to 15Mb per upload, which is enough to handle most file types.
If you want to edit the title of a file just click on the text and type your changes, or if you wish to remove a files simply click on the delete link next to the file name and confirm when your browser asks you to.
Learn how to: