For top-level server adminstators:

Log into your SmarterMail Admin Interface
Double-click on the domain which holds the user that you want to change the password for
Click the Manage button at the top of the screen
The Impersonation box will pop up with an address in it.
Click the Impersonate button
Double-click on the user who's password you would like to change
Click the ellipsis (...) icon at the top of the screen and then click Change Password

For domain administrators:

Log into your webmail account
Click the square icon (9 small squares) at the top of the screen
Click Domain Settings
Click Accounts
Double-click on the user who's password you would like to change
Click the ellipsis (...) icon at the top of the screen and then click Change Password

For regular users:

Log into your webmail account
Click the square icon (9 small squares) at the top of the screen
Click Settings
You will now see a Change Password button. Click it to change your password.