This is a mess. Where did you get this code?
1. It is declared as HTML5 doctype but why at the same time
<html xmlns="http://www.w3.org/1999/xhtml">
2. There is this jQuery library, but you are using plain js code inside the jquery document.ready() method.
Do read out:
HTML Tutorial[
^]; and
jQuery Tutorial[
^]
Coming back to your question, if you want some action to take place (e.g changing the value interval to that in a text box) upon the happening of some event (e.g. changing of value in the text box), then you have to first bind that action to that event. As it is now, they are not bound. Check these out:
JS onchange Event[
^]; or
jQuery .change()[
^]
After you have updated the interval value, clear the current timer, then start the timer with a new interval, how to do this? Ask Google.