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