If you followed the tutorial, you must have created a StaticSidebar.ss. Most of the things you see in your sidebar stem from there. Look for:

and don't forget to ?flush=1

I already changed that <h3>Static Sidebar</h3> -part (which I assumed to be the right place, but was confuced when it didn't change anything), but can you explain what does that ?flush=1 means ?

And yes, I'll check the template again... and if don't find, I'll do it again.. Maybe I'll find the missing part (or the part that's left)

The ?flush=1 means to flush the site's cache.
Taken from the docs: "Whenever we edit a template file, we need to append ?flush=1 onto the end of the URL, e.g. http://localhost/home/?flush=1. SilverStripe stores template files in a cache for quicker load times. Whenever there are changes to the template, we must flush the cache in order for the changes to take effect"

Here is the link: http://doc.silverstripe.org/sapphire/en/tutorials/1-building-a-basic-site