How do I access a UDataTable from C++?

I have Engine/DataTable.h included, and here is the declaration.
UPROPERTY(BlueprintReadWrite, EditAnywhere)

UDataTable* ItemTable;
This is a value in a UUserWidget. How do I fix this?

I am not sure what you want to achieve, you can access UDataTable* just like you would access any other pointer. Do you mean reading the actual data in the data table?
Here is a nice article about that: