To update your kernel you can use a guide like: https://wiki.ubuntu.com/Kernel/MainlineBuilds (but you could just google for it)
Installing upstream kernels
To boot into an upstream kernel you only need to download and install the image.deb package that corresponds to your architecture. In addition, if you need to build any external modules you also need the correspinding headers-_.deb and headers-_all.deb packages.
For example, let us use http://kernel.ubuntu.com/~kernel-ppa/mainline/v3.14.4-utopic/.
If you were testing i386, you would want those marked A.
If you were testing amd64, B.
(Use generic kernel unless you have a special need for lowlatency kernel, such as audio production.)
B linux-headers-3.14.4-031404-generic_3.14.4-031404.201405130853_amd64.deb
A linux-headers-3.14.4-031404-generic_3.14.4-031404.201405130853_i386.deb
linux-headers-3.14.4-031404-lowlatency_3.14.4-031404.201405130853_amd64.deb
linux-headers-3.14.4-031404-lowlatency_3.14.4-031404.201405130853_i386.deb
AB linux-headers-3.14.4-031404_3.14.4-031404.201405130853_all.deb
B linux-image-3.14.4-031404-generic_3.14.4-031404.201405130853_amd64.deb
A linux-image-3.14.4-031404-generic_3.14.4-031404.201405130853_i386.deb
linux-image-3.14.4-031404-lowlatency_3.14.4-031404.201405130853_amd64.deb
linux-image-3.14.4-031404-lowlatency_3.14.4-031404.201405130853_i386.deb
Once you have those downloaded they will need to be installed using dpkg:
sudo dpkg -i *.deb
When this process completes you should have a new entry on your boot menu representing the upstream kernel. This will appear as an entry like this:
Ubuntu Trusty, kernel 3.14.4-031404-generic
Updating Gnome to 3.12:
This step is taking from: http://linuxg.net/how-to-install-gnome-3-12-on-ubuntu-gnome-14-04-trusty-tahr/
$ sudo add-apt-repository ppa:gnome3-team/gnome3
$ sudo add-apt-repository ppa:gnome3-team/gnome3-staging
$ sudo apt-get update
$ sudo apt-get install bijiben polari gnome-clocks gnome-weather gnome-maps gnome-music gnome-photos gnome-documents gnome-contacts epiphany-browser gnome-sushi gnome-boxes gnome-shell-extensions
Now you should logout and log back in to Gnome 3.12.