Password changes are handled by the subversion server administrator. As a user there is no password change option.
Check with your server admin.
If you are the admin, find your SVN Server installation. If you don't know where it is, it could be listed in Start->Programs, running under services in Start->Control Panel->Services or it could be listed under C:\Program Files.
The SVN Server should have an application to run to add/change/delete authentication and users.