2 Answers

Try here...http://www.cultofmac.com/272334/reset-password-without-system-disk-os-x-tips/

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Question: 

How can I change the admin password when I do not know the password. I have tried re-booting holding down command R but I still need the old password to change it to a new password. Can you help?

I cannot download from the internet because I need a password with my user name. When i use my password it does not work. I need to reset the admin password but I cannot because you need the original password to change the password. Can you tell me how to change the admin password when you don't know the old password?? What can I do I have tried re booting and holding down command R but you still need the original password.

iMac, OS X Yosemite (10.10.3)

Answer:  

Reset Password

OS X 10.10 Yosemite 

Start the computer, then press and hold down command and R keys to start into recovery partition.

When you see the Apple logo, release the keys.

Wait until OS X Utilities window shows up.

Move the mouse to the menubar at the top and click "Utilities", then select "Terminal" from the drop down.

Terminal window will appear.

Type in resetpassword and press enter on the keyboard.

Leave the Terminal window open.

Reset Password Utility window will open with Macintosh HD selected.

Select the user account from the popup menu box under “Select user account”.

Enter a new password.

Reenter the new password for the user.

Enter a hint.

Click the "Save" button.

Click the apple icon in the menubar and select Restart.

Log in.

If Keychain dialog box appears, select “Create New Keychain”.