First of all, you don't change url directly and you need permissions for tabs in your manifest file, to have access to tabs API
add this to manifest.json file:
"permissions": ["tabs"]
Then when you wan't to change url in active tab, you have to do it in two steps,
get active tabId and inside of callback function you can change url, like so:
chrome.tabs.query({active: true}, function(tabs){
chrome.tabs.update(tabs[0].id, {url: "your url here"}, function(){});
});
Anyway you should take a look at chrome's API:
https://developer.chrome.com/extensions/tabs#method-query
Hope that helps. 😃