1) Login to the web interface as administrator through URL : mail.yourdomainname.com with administrator username with password
Note: The email account need to have administrator privilege in order to reset email account password from Smartermail
2) Go to the last button of the top panel > Domain Settings
3) Select the Accounts on Left panel
4) Click on the email account from email account list then click on ... button and Change Password
5) Change the password and save it.