An example update using CURL:
curl -H 'Authorization:Basic $AUTHSTRING' "https://nic.changeip.com/nic/update?&hostname=example.com&ip=1.2.3.4"
Replace $AUTHSTRING with a base64 encoded value of your username and password in an authentication header.
We would suggest using this form to generate the value (although we do not author nor support it):
https://www.blitter.se/utils/basic-authentication-header-generator/