Many people liked the classic look of the compose window in Gmail.
If you prefer the that look, there is a Chrome extension called Old Compose that you can install to restore the classic look in GMail.
Once you install the extension, the Old Compose icon will be found in the upper righthand corner of the browser window. Clicking on the icon (circled in red) allows you to toggle between the two different classic versions: Classic Compose or Improved Compose:
As with every Chrome extension, you can always disable it or delete it.
Check out other Chrome extensions in the Chrome Webstore.