I've forgotten the password for my MacBook Pro, which uses OS X 10.9.1. How can I reset my password?
The simplest way may be to use the Recovery HD, but this won't work if you have FileVault enabled.
Boot into the Recovery HD (hold ⌥ alt/option at boot and select the Recovery HD).
Once it has booted, click Utilities, then Terminal.
resetpasswordthen ↩ enter.
Follow the instructions.
Restarting Setup Assistant
In addition to using the Recovery HD, you can also restart the Setup Assistant, create a new Administrator account, then use that account to reset the password to the original account. This process will work if you have FileVault enabled, although you'll have to unlock your drive first before mounting.
You can restart the Setup Assistant by performing the following:
Boot into Single User Mode (hold ⌘ cmd + S at boot).
Mount the drive by typing
/sbin/mount -uw /then ↩ enter.
Remove the Apple Setup Done file by typing
rm /var/db/.AppleSetupDonethen ↩ enter.
Reboot by typing
rebootthen ↩ enter.
Continue setup, making sure not to transfer data.
Setup a new administrator account. You can use this account to reset the password to the original account (through System Preferences → Users & Groups).
The new account can then be deleted once the original account has been recovered.
The easiest way to reset a user password is following the help menu on Finder on the Mac itself.
Search for "Reset a login password" and you will be guided to the steps needed whether you use FileVault or not (as well as have help to explain how to tell if you have FileVault enabled).
Additionally, Apple's guide on resetting passwords is:
Depending on how you secured the Mac there are several avenues to unlock things. If you have skill with terminal, this thread might also be useful:
Resetting User Account Password (10.7 +)
1) Boot into your Recovery Partition holding down command + R at the startup chime
2) Select 'Terminal' from your Utilities Menu Bar
3) Type the following bolded command in: resetpassword
4) From the window that appears, select your boot volume and the appropriate user account
5) Type in the new password and confirm the new password
6) Click Save, close this pane and quit Utilities
7) Select your boot volume when presented with 'choose startup disk' and then reboot your machine