how can i update my new ipad mini to ios 11.3.1? the latest update on it is 9.6. ni more updates.

Sounds like you have a first generation iPad Mini which is not capable of running any version of iOS beyond 9.6.

If you have need to upgrade beyond 9.6 then you need newer hardware to run it.

In Settings App, General, About, please post your iPad Mini model ID number so we can tell what iPad Mini model you have and can advise accordingly.