There is a drop down at the top right of the editor navigation showing the current logged in username, in that, click "My Settings"
The direct url would be %pathtoepiserver%/CMS/edit/mysettings.aspx
e.g
/episerver/CMS/edit/mysettings.aspx
EDIT:
Just realised you said external users, not CMS editors. There's no built in way with EPiServer, but the logins use ASP.Net membership system, which has its own controls and templates for common membership functions.
This article might be of assistance:
http://www.asp.net/web-forms/tutorials/security/admin/recovering-and-changing-passwords-cs