How can I update my gnome to gnome 3.18?

Not for 15.04. You need Wily, available since of today, and at least one PPA

sudo apt-add-repository ppa:gnome3-team/gnome3

To get more bleeding edge versions of the components, additionally

sudo apt-add-repository ppa:gnome3-team/gnome3-staging

Update the package database and upgrade

sudo apt-get update
sudo apt-get dist-upgrade

And if you have problems, start the commands below to go back to the version of the official Ubuntu repositories

sudo ppa-purge ppa:gnome3-team/gnome3
sudo ppa-purge ppa:gnome3-team/gnome3-staging

At this moment, you can install Gnome 3.18 on Ubuntu Gnome 15.10 Wily by the PPA ppa:gnome3-team/gnome3-staging:

sudo apt-add-repository ppa:gnome3-team/gnome3-staging

If you want update to Gnome 3.18, you'll need upgrade to Ubuntu 15.10.

On Ubuntu 15.04 you just can update only to Gnome 3.16, the actual version bundled with Ubuntu 15.10.

Follows the step by step valid to update to both versions:

http://www.omgubuntu.co.uk/2015/06/how-to-upgrade-to-gnome-3-16-in-ubuntu-15-04