How to install a new kernel?

Ubuntu 16.04 comes with keel 4.4 series. That 3.18 keel is very old. It was released by Linux keel.org in 2014 and the Linux developers will drop support for it January next year.

https://www.keel.org/category/releases.html

This is how you do it

https://wiki.ubuntu.com/Keel/MainlineBuilds

If you are using a 64 bit CPU you need to download files marked B & the file mark AB. But here is the problem that you need to overcome. The Ubuntu keel team does not provide a 3.18 keel. Ubuntu 14.04.2 came with keel 3.16 & Ubuntu 14.04.3 came with keel 3.19. You will have to find a source for the linux-headers 3.18 ... amd64.deb & linux-headers 3.18 ... all.deb & linux-image 3.18 ... amd64.deb.

You need those 3 files - headers_amd64.deb; headers_all.deb & image_amd64.deb relevant for the keel you want to install.

Regards