I'm using elementary Freya.
Is there any way to upgrade the Linux kernel to 4.0?
And by upgrading kernel can we download and install packages from elementary ppa ?
Download the following:
cd into the download directory and run
sudo dpkg -i *.deb
On reboot your system should boot the latest kernel. You can test this by running
uname -r. Note that this will not automatically update from this point forward (in particular, you won't get any security fixes), and that this is not supported by elementary.
As for your question about PPAs: installing a newer kernel will not change anything about your ability to use PPAs.