One of the change was new Gmail compose box in which when you click on Compose button you will be served with a minimal small pop-up box at side in which you can compose email.
This interface is accepted by many but some folks wants to switch to old interface, if you too wants to switch back then here is useful Google Chrome Extension which lets you switch back to Old Gmail Compose Interface.
Note :- As it’s a free extension developers has made it a tough call to download, you need to provide your email address, tweet it or share on Facebook before download link appears. But some folk has uploaded it HERE which lets you download.
Once you had extension open chrome’s extension page by typing :-
Now drag and drop download extension and install it, once installed open you Gmail and compose an email and you will be served with old Gmail Compose interface.