Hi Danng,
It is always a good idea to make sure you have a good backup of your current configuration before performing an update. If you have already done so, you should be able to restore your current settings in the case of any issues:
iOS: How to back up
http://support.apple.com/kb/HT1766
For an outline of the steps for updating an iOS device, the following article may be useful:
iOS: How to update your iPhone, iPad, or iPod touch
http://support.apple.com/kb/HT4623
There is also an article for addressing specific errors you may encounter during an update:
iOS: Resolving update and restore alert messages
http://support.apple.com/kb/ts1275
Regards,
- Brenden