Change Your Password
To change your Linux desktop password go to About Me by selecting
Ubuntu Main Menu --> System --> Preference --> About Me
In the About Me window, click on the Change Password button located in the upper left-hand corner of the window
Enter your current password in the highlighted field, and click authenticate
Enter your new password in the "New Password" field, and again in the "Retype New Password Field"
Click on the "Change Password" button If your password has been successfully changed, all the fields will be cleared, and you will see a "Your password has been successfully changed." notification below the "Retype New Password Field".