How o I change the passwor to my router?

I have a Linksys router an I nee to change the password to it because my sister which lives 2 floors own from me got a hold of it and has her laptops and iPod's on it plus for some reason when my HP Touchsmart PC was set up the HP tech made my internet wifi so my internet signal comes directly from my router. On top of that we have the Xbox 360, 2 laptops, an iPod and 3 DSi's all connected on the same tiny router and I'm frustrated because I'm barely getting internet or it's way too slow. Like I was trying to stream a movie from Netflix and it wouldn't play the movie because it said the signal was too weak. So, how can I chane my routers password so no one can use it and will this help??? Thanks for your replies.