Refer to the documentation for httpd auth module you are using. You change the password the same way as you created it. This is usually named htpasswd, htpasswd2 or similar.
How to make this user self-servicable is out of scope for Subversion as it uses common and adaptable auth methods, including directory services.
Andreas
No, but I might at this point note that you /basically/ were unable to grasp that authentication in mod_dav_svn is delegated to Apache httpd. In this case, there simply is no such thing as a "Subversion password", only one for httpd.
And for self-service password changes Subversion does not care how you do it. But it will be a feature of one of the many httpd auth methods and modules, not mod_dav_svn. You were given several pointers, but without grasping the separation that may not help you.
Andreas