Step 1: Open your minecraft.jar file in Winrar
Step 2: Open the lang folder
Step 3: Find and open the en_US file
Step 4: Under edit click find and type in what you want to rename
(If it does not find it, under the typing space there are two circles you can check, change the one that is checked and then press find again.)
Steps 5: Find the item you want to change the name of and change the name of it, but only where it says the name after the equals sign.
EXAMPLE:

I want it to call cookies muffins

4: Type in cookie
5: It says Cookie.name=Cookieitem I have to change Cookieitem to Muffinitem

NEVER change 'Cookie.name' and don't change the 'item' part after the second Cookie.