My apologies. I meant starting up the computer or re-starting. You can follow these instructions with which you should be familiar:
Boot from the Recovery HD
- Restart the computer.
- Immediately, at or before the chime, press and hold down the "COMMAND" and "R" keys together.
- Release the keys when the Apple logo and progress bar appear.
- Upon completion the Utility Menu should appear.
The Utility Menu appears on a gray screen. There is a menubar on the screen. The Utility Menu has four selections: Restore from Time Machine backup; Install macOS; Get help; and, open Disk Utility. In the lower right corner of the Utility Menu is a button labeled, Continue. You can open any of the options either by double-clicking on the option or by selecting the option and clicking on the Continue button.
So, to do what you asked:
Install El Capitan or Later by Erasing the Disk
If possible, back up your files.
- Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
- Select Disk Utility from the Utility Menu and click on the Continue button.
- When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
- Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
- Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
- Click on the Apply button, then wait for the Done button to activate and click on it.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.
If you don't want to erase the disk, here is how you reinstall macOS over an existing system without erasing the disk:
Reinstall El Capitan or Later Without Erasing Drive
Please be sure you back up, if possible.
- Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
- Select Disk Utility and click on the Continue button.
- Select the indented (usually, Macintosh HD) volume entry from the side list.
- Click on the First Aid button in Disk Utility's toolbar. Wait until the Done button activates, then click on it.
- Quit Disk Utility and return to the Utility Menu.
- Select Install OS X and click on the Continue button.