I am testing this manually:

I currently have the correct IP linked to my domain within Dynu, as a test I changed the IP manually to something bogus and run the update URI from firefox (manually for the time being)
e.g.

https://api.dynu.com/nic/update?username=MYUSER&location=HOME&password=xxxxx

for some reason I get the nochg message back and looking at the Dynu account page I can see the IP is not updated. Am I missing anything?

Thanks

nochg means the domain already maps to the IP address from where the IP update is sending from.

You cannot change the IP resolution manually in the control panel and then perform an IP update as the update system cannot detect that you manually change the IP. If the IP update URL runs from a different network, the IP will get updated with 'Good' response.