All password corresponding to all account on Mac OS X all version including El Capitan, Yosemite, Mavericks,… can be reset when you boot your Mac from the Recovery Partition on the Mac’s hard drive.
How to Boot Mac OS X into Recovery Partition to reset password?
Just restart the Mac, then hold download both Command and R keys until you see the startup screen. You then will see the OS X Utilities screen.
Drop down the Utilities menu, then select Terminal
In a new window prompted, type ‘resetpassword’ (hit enter)
The step 3 will bring you to the Reset Password screen. You now just choose the Volume Hard Drive in the top part of the window pane, then for the user that you need to reset select the user account in the lower part then add and confirm the new password and click save, you get a confirmation screen and details about the keychain account.
No more sad days, you now can restart the mac (Drop down the Apple Menu and click Restart) to boot into the modified account with the new password.