I cannot seem to install iOs5 on my 4.1 iPhone.

Question

I have a second hand iPhone 4 that I bought, principally, to use for the Airbnb App. Now they have told me to update, which I did, and it tells me it requieres iOS 5. I have tried to download it directly from the App store but it will not even give me the option.
The phone is connected to the keyboard by USB and I cannot do it via the website either.

Answer 1

If you are currently on iOS 4 then you will need to update via your computer's iTunes (if your computer's iTunes is not up-to-date then you may need to update that first).
Connect the iPhone to your computer's iTunes and copy any purchases off the phone to your computer via File > Transfer Purchases - you may also want to copy photos and any important documents off the phone as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case. You can then force a backup of the iPhone by right-clicking the iPhone 'Device' on the left-hand side of iTunes and selecting 'Backup'.
You can then start the update by selecting the iPhone on the left-hand side of iTunes, and on the Summary tab on the right-hand side clicking the Check For Updates button.
Updating to iOS 5+ : http://support.apple.com/kb/HT4972