W: This may break your system, so proceed with proper caution

if you want a full custom upgrade/update of your system without a fresh install then follow the steps below:

#1. first the repository list needs to be updated.

for upgrading to LM 16 petra (the current latest) the following modifications are needed (to get the latest repository list manually):

open /etc/apt/sources.list.d/official-package-repositories.list

gksudo gedit /etc/apt/sources.list.d/official-package-repositories.list

a. replace the LM names (say, olivia for LM 15) with the latest LM name (petra for now)

b. replace the ubuntu names (say, raring ) with the latest one used in latest LM version (say, saucy).

now save & exit the file.

open /etc/apt/sources.list.d/official-source-repositories.list

gksudo gedit /etc/apt/sources.list.d/official-source-repositories.list

a. replace the LM names (say, olivia for 15) with the latest LM name (petra for now)

b. replace the ubuntu names (say, raring ) with the latest one used in latest LM version (say, saucy).

save & exit.

#2. now your packages need to be updated:

sudo apt-get update

sudo apt-get dist-upgrade

now all of your softwares are new versions along with dependencies.

#3. Update your kernel.

find the kernel packages in synaptic and upgrade them.

kernel packages:

linux-headers-VERSION-NUMBER_all.deb
linux-headers-VERSION-NUMBER_amd64.deb or linux-headers-VERSION-NUMBER_ix86.deb
linux-image-VERSION-NUMBER_amd64.deb or linux-image-VERSION-NUMBER_ix86.deb
linux-image-extra-VERSION-NUMBER_amd64.deb or linux-image-extra-VERSION-NUMBER_ix86.deb # if available # x is a number

in this process all of your softwares along with kernel will be updated to the newest version supported by LM.

the testing releases can also be checked out by this process. for that the ubuntu name needs to be replaced by the latest testing ubuntu version name available (say, trusty). It's extremely dangerous and has a high potential to break the entire system.

But recently i have checked even the unstable trusty version (not a full upgrade, only several softwares) and rolled back to saucy (LM 16 cinnamon) only to check out some of the latest softwares and nothing happened to my system. but upgrading the whole system is another thing. so proceed with caution...