Change Password using apex code

setPassword

ID userID String password

Void

Sets the password for the specified user. When the user logs in with this password, they are not prompted to create a new password. Use resetPassword if you want the user to go through the reset process and create their own password. Warning Be careful with this method, and do not expose this functionality to end-users.

e.g. System.setPassword('006A123ahjklkk','password123');