Iphone 3gs upgrade from 5 to 6 is free?

I have iphone 3gs with Ios5 version. I want to upgrade from ios 5 to ios 6. It is free to upgrade from home? If yes any problem arrive after upgrade?

It is safe to upgrade from home OR i have to go mobile shop for upgrade. & how much cost for upgrade.

I need Help. Thank you very much in advance.

Can i use the following link for upgrade in my iphone 3gs?

http://www.midpac.edu/it/downloads/how-to-upgrade-to-ios6.pdf

Its simple procedure.
Connect Your 3gs with your Pc
Open itunes and click restore make sure you have Wi-Fi to download ios6 file. Its a huge about 700mb file.
After download is finish itunes will do all the steps with its own.
It may take 25-30min to upgrade your 3gs
make sure that your 3gs is factory unlocked otherwise your will be locked
if you don't have Wi-Fi go to repair shops they may upgrade it at rs 250-300.