If i understood it right, you are talking about buttons inside your app. If its a button, you can change the image as shown in the image below. You question is not very clear so provide more info if you meant something else.
Update: I use this free tool for icons. http://appicontemplate.com/ios7 to make my icons which gives me all sizes i need. I add all files to my project image assets. you can add the file names as shown in the apple document.