Yes, always back up first. That is a general rule, as it is always possible that things will go wrong. I would hate to lose all my data, it represents a lot of work!!

Then just run preupgrade.

Read about it here:

http://fedoraproject.org/wiki/PreUpgrade

would preupgrade keep my fedora 15 setting & windows partition intact?
I'm planning to back up important docs and purchased music only...i won't be able to back up my movies and other downloads for now...
what are the chances of losing all data in %age ?

Yes, your win partition won't be touched. If you do not have much data, you can also back it up to some dvds. I use an external 320GB hd. I can just manually run all my stuff onto it, without any problem.

Some things in Fed 15 will be changed, but the basic look and feel will stay the same.

When you first installed F15 did you let Fedora do your partitioning or did you do a Custom Partitioning ?

The reason I ask that is if you made your own custom partitions for example;

/dev/sda? / 40% of your allowable disk space to install Fedora
/dev/sda? /home 60% of your allowable disk space to install Fedora
/dev/sda? swap 1024mb

If you made similar cutom partitioning You would do fresh install of Linux OS in / (by Formatting this partition)

The /home partition would not be Formatted and you would be able to save personal and hidden files and they would work in new install.

Upgrades will never work, because of conflicts between old and new files in /. I have tried upgrades a number of times and things just didn't work.

SO your best bet is to do a fresh "Custom Install" in that way your /home/user and all your files will not be affected on each Fedora upgrade.

1) as i have a multiboot configuration, do i have to do some changes in grub menu to upgrade using preupgrade?
2) I've got only 3.5GB of free space on this laptop, is it sufficient for preupgrade to work?
3) can i use my 4GB USB flash drive to upgrade fedora? will it keep my data and settings intact?

I believe any installer will rewrite grub.conf. Look in /boot/grub or maybe /boot/grub2, it's there. I often edit grub.conf directly, although it says don't do that. This is because the grub.conf I have was written by Ubuntu. When Fedora updates the kernel, it writes to the Fedora partition /boot/grub2/grub.conf Ubuntu will never see this!! So I just copy the new entry into the grub.conf of Ubuntu! May not be the right way to go, but I don't know any other way, and it works!!

3.5 GB is probably too little, it may work, but as soon as you put a video, or a few photos, it will be full. Try to free some up, give yourself at least 20GB, and you need a swap partition if you don't already have one. Don't expect preupgrade to work, it probably won't.

You can install Fedora on your usb stick. Then boot from there. If you put the netinstaller (if there is one) on there, it won't take up much space. I can't boot from usb, although the bios is set to do that. Don't know why.

I have given up on Fedora at the moment. It had too many problems, so I am using Ubuntu. I'll try Fedora again when 18 is ready.