It's already the default for users.
Gmail's new compose now default
You are busy people, so it's no surprise that an overwhelming number
of you opted to try out Gmail's faster, simpler compose experience
after it launched last October. In addition to telling us what you
love about the new compose experience (like how much easier it is to
multitask!) you’ve also been sending us helpful suggestions for what
features you'd like to see added. As a result of your input, we're now
ready to introduce the new compose experience as the default for
everyone. We're looking forward to hearing what you think!
If you're already using the the new compose experience, you've
probably noticed it's gotten a lot of upgrades over the last few
months, from a new way to send files with Google Drive to
much-requested features like pop-out replies. We’ve also added support
for originally missing features like starring and labeling when
composing and the Canned Responses lab.
The new compose will be rolling out to everyone over the next few
days. You can learn more in our help center, or follow us on Google+,
where we'll be posting more tips for using the new compose.