This is an old thread, but I just had the same issue on Windows and the accepted answer didn't work for me, so I'm posting this for others who come along.
You can check your version by opening your Docker Quick Start terminal and running:
$ docker --version
Docker version 17.10.0-ce, build f4ffd25
To update all you had to do is run $ docker-machine upgrade
User1234@HP-1234 MINGW64 ~
$ docker-machine upgrade
Waiting for SSH to be available...
Detecting the provisioner...
Upgrading docker...
Stopping machine to do the upgrade...
Upgrading machine "default"...
Default Boot2Docker ISO is out-of-date, downloading the latest release...
Latest release for github.com/boot2docker/boot2docker is v18.01.0-ce
Downloading C:\Users\User1234\.docker\machine\cache\boot2docker.iso from https://github.com/boot2docker/boot2docker/releases/download/v18.01.0-ce/boot2docker.iso...
0%....10%....20%....30%....40%....50%....60%....70%....80%....90%....100%
Copying C:\Users\User1234\.docker\machine\cache\boot2docker.iso to C:\Users\User1234\.docker\machine\machines\default\boot2docker.iso...
Starting machine back up...
(default) Check network to re-create if needed...
(default) Windows might ask for the permission to configure a dhcp server. Sometimes, such confirmation window is minimized in the taskbar.
(default) Waiting for an IP...
Restarting docker...