It is not a good idea to use the Root account to manage and work with your AWS account.   Ideally you have setup IAM user accounts with only the required permissions.

However occasionally you need to update your Root level account password.   This video quickly shows you how to do it

(if this video is low quality,  try opening it full screen and playing it again,  the video is short and may be done playing before YouTube can catch up with downloading a higher quality version)