What to do if El Capitan will not install

• Go to the Applications folder
• Open the program called “Install OS X El Capitan”
• Follow the on-screen instructions
• If the program is not there or fails to install then move it to the Trash and empty the Trash, then  re--download from the Mac App Store.

Full credit for this post is due to Topher Kessler of MacIssues.com