I am currently using Ubuntu 15.10 I will be upgrading to ubuntu 16.04, Since I have a lot of software installed, after upgrading will I lose all of them? If so is there a way to backup all these software before upgrading, so I do not have to download all of them again. Thanks :)
If you upgrade using any of the in-built Ubuntu mechanisms then please follow this answer. Using these methods Ubuntu will download all the packages for you and then, upgrade your system. So, you will not loose any installed softwares. But keep in mind that this method is a bit risky, as it has been seen that many a times upgrades fail, leaving the system in an non-usable state.
If you are aiming for a clean install, then follow the below instructions. In this method you will loose all your installed softwares.
apt-get downloads the packages for installation, it downloads those in the directory
/var/cache/apt/archives/. So, until and unless you have manually deleted those files, they should be there. Copy those files from those directory and install them onto your new system:
cd /path/to/the/debs && sudo dpkg -i *.deb
BUT REMEMBER THAT THOSE PACKAGES MIGHT BREAK SOME OF YOUR PACKAGES IN 16.04 AS BECAUSE NEWER VERSIONS OF THESE PACKAGES MIGHT BE NEEDED IN XENIAL AND YOU ARE DELIBERATELY INSTALLING THE OLDER VEDRSIONS.
P.S.:- There are some very good softwares that helps an Ubuntu user in upgrading from one version of Ubuntu to other such as Aptik, etc. These softwares make a list of your installed softwares so that you don't have to manually download them again. Just run the software and it will download and install them for you.