I'm using two Samsung mobiles: Samsung Young and Samsung Ace Duos. I am getting software upgrade messages. Is it safe to upgrade software and will it harm my data, contacts, messages and important details saved in phone memory? Please help me out. Do tell me the entire procedure to save my data while updating.
Usually OTA / Kies updates do not wipe your data, but they MIGHT. Try backing up with MyBackup on the Play store. There is a version for root and a version for NON rooted phones. It will back up contacts (possibly already on google), SMS, MMS etc etc, on to your SD card. To be totally sure there is no harm in copying the backup onto your PC just incase the worst happens.
If you are rooted get Titanium backup and you can backup all your apps and data just in case. It is often recommended to do a factory reset after an OS update, but personally I've never needed to do that.
And read up on what to do when updating, make sure you have your battery over 50% at the very least. Some manufacturers recommend NOT having the phone charging while updating but AFAIK that's mainly HTC's recommendation.
Just to add to RossC's excellent answer - you can do a full backup via Kies first to make sure you're safe: http://www.samsung.com/us/support/SupportOwnersFAQPopup.do?faq_id=FAQ00029017&fm_seq=29185
There have been reports of the restore failing, although it seems there is a workaround: Cannot restore Kies backup after firmware upgrade