This step-by-step tutorial will teach you how to reset a lost or forgotten user account password in Mac OS X without the use of installation discs.
Note that this procedure should only be used as a last resort. Prior to this operation, it is recommended that users be familiar with the password and security documentation provided by Apple.
How To Reset OSX Password with the Rescue Partition
Recent Mac models are shipped without installation discs (which usually contain the password reset utility).
Instead, all the installation data and utility tools are stored on a hidden partition, commonly referred to as Rescue Partition.
Reboot your Mac while holding the [CMD] + [R] keys down
A window titled, OS X Repair Utilities will be displayed
Click on the Utilities > Terminal and run the resetpassword command to open the password reset utility
Select the system drive and then choose the desired user account
Enter the new password and confirm
Click the Save button to apply your new settings
Before exiting the password reset utility, move to Reset Home Directory Permissions and ACLs section and click on the Reset button
Close the Terminal and exit the Repair Utilities screen. Restart your Mac and sign in with your new password
More Information
Apple provides a variety of resources that can help you recover your system passwords.
More information can be found in the links below.
How To Reset OSX Password using Your Apple ID
How To Recover a Lost or Forgotten Password in OS X El Capitan
Reset a Login Password in OS X El Capitan
