Do you understand more generally how to install software using the package
manager?
https://trisquel.info/en/wiki/add-and-remove-programs
Go to the settings, select "synaptic package manager" and then search for
"linux-image" and you should see a list of possible kernels, one of which is
Ubuntu's 4.2 kernel.
Alternatively add jxself's Apt repository using the instructions that Magical
Banana gave you. This will be the latest kernel, newer than version 4.2, but
I think that 4.2 should be suitable.
