Writing Head In Span At Runtime ?

I'm building a shopping website, in which there's a select where the user select which products to show. beneath the select, there should be a label of the product followed by a table written at runtime of the result. My problem is is the title so far, the designer gave me the prototype with a static title within a span, when i try to call the function to change the heading, it's written in a new blank page not within the same place.

Here's the code so far:

HTML Code:

I was thinking that it's because of document.write and it's how it works, but i don't know how to write to the same document in the same place of the script.