If you have everything already in iTunes and you have synced with iTunes at least twice (to assure you have backup) then you should not lose anything. Upon completion of the update you will have an option to restore the device from a backup.
But, the update process will erase everything until you restore your backup.
Vital Resources For Your iDevice (Updating, Apple ID, and iCloud)
A. Upgrading iOS
4. iPad Support
a. Updating Your iOS to Version 6.0.x from iOS 5
Tap Settings > General > Software Update
If an update is available there will be an active Update button. If you are current,
then you will see a gray screen with a message saying your are up to date.
b. If you are still using iOS 4 — Updating your device to iOS 5 or later.
c. Resolving update problems
1. Apple ID FAQ
E. User Manuals
3. iPod Touches