Q: How to update my ipod when my ipod is on ios 4.3.5?

Sounds like this problem:

Update Server

Try:

- Powering off and then back on your router.

- iTunes for Windows: iTunes cannot contact the iPhone, iPad, or iPod software update server

- Change the DNS to either Google's or Open DNS servers

Public DNS — Google Developers

OpenDNS IP Addresses

- For one user uninstalling/reinstalling iTunes resolved the problem

- Try on another computer/network

- Wait if it is an Apple problem