changepw.html: Simple HTML form. Put it into your ~/html for example. Change XXXUSERXXX and XXXHOSTXXX according to your uberspace user and host name.
changepw.py: Put it into your ~/cgi-bin and give all users executable rights (chmod a+x ~/cgi-bin/changepw.py).
Python script will check if user exists, if new password and repeated new password match and also if old password is correct.
Dependencies of Python script: python-cdb
Install on your uberspace with
easy_install-2.7 python-cdb
This script probably could be extended to allow each IP only 5 password changes per day to avoid misuse.