If you are on Windows the easiest way to delete blockchain_headers it to copy/paste %APPDATA%\Electrum in your windows explorer, and it will lead you to Electrum folder. There you can delete this file and then open Electrum again, depending on your internet speed it should be synchronized in minutes or even less then minute if you have fast internet (file is just 40 MB+).

In case you are on Mac or Linux :

On Mac:

- Open Finder
- Go to folder (shift+cmd+G) and type ~/.electrum

On Linux:

- Home Folder
- Go -> Location and type ~/.electrum

http://docs.electrum.org/en/latest/faq.html