Can't change admin password

I have been able to login to my admin website and change my wifi password to something very strong, however I cannot complete the process for changing my admin password. I complete all the requests made and when I click apply/next/continue, nothing happens.

Someone has cracked into my wifi network and is using about 50GB/day. I need to change the admin password securely and then fix a new wifi password.

Any ideas, please?