You cannot. The last version of iOS for a 2G Touch is 4.2.1. If you are running 4.3.3 then you don't have a 2G model, but a 3G model. In that case see:
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