Use the Change Password button on the bottom of Retrieval's Configure screen
You will be presented with several text boxes to type in your old password and your new password.
You need to type your new password twice to guard against typos. Password changes need to filter down through the system, so please allow about ten minutes before you make a request with your new password.
If you have forgotten your password, you need to contact the archive hotseat by emailing them at archive@stsci.edu or calling 410-338-4547 during east coast working hours to change your password.