Cant upload files like pdf when adding items

I tried uploading a pdf file to an item and got this big error. what should I do please.

What version of Omeka are you using?

Version 3.1.1, the latest Omeka Classic version, should fix this problem, which is related to your host disabling the exec function.

Thank you Jflatnes. How can I update a live Omeka site??
I am currently using version 3.0.3.

There are instructions for upgrading in the manual.