Unless you have access to another Mac or a working OS X on the Mac you're talking about, you can't reinstall El Capitan over itself.As per KMT's answer, if you can boot into Recovery Mode and you have a Time Machine backup then your best option may be to restore to that.Otherwise, if you just want to wipe your Mac and start again then this support page that the Public Beta site references for those who want to revert back from the Public Beta to a stable release version provides the following solution:You can use Disk Utility to erase the disk before reinstalling. If you’re installing on a notebook computer, make sure your power adapter is connected and plugged in. Use these steps if you need to erase a disk before installing OS X:Before you begin, you need to be connected to the Internet.Choose Apple menu > Restart, and then hold down the Command (⌘) and R keys while the computer restarts.Select “Disk Utility,” and then click Continue.Select your startup disk from the list on the left, and then click the