How to change bit rate/file size from things downloaded from Itunes store?

For some reason, my computer has been downloading things at a higher bit rate than normal, so a average song is taking up twice the space it used to. How can I change this?How to change bit rate/file size from things downloaded from Itunes store?
Why do you want to change it?

Anyways, try audacity or something, you need to convert it.