Updating to iOS 7 from 4.3.3?

I have a few questions here please answer than with question numbers.
1.Is there anyway to download iOS 7 separately and then install it to iPhone 4 (iOS 4.3.3)?
2. What happen if accidentally disconnect Internet or idevice from iTunes while updating? How to recover from it?
3. I have a slow Internet connection which has a speed of 15kb per sec downloading is it a problem?

Please answer them all. I have jailbroken iPhone which is iOS 4.3 which has no software update option in settings.

I just have to connect to iTunes and then click check for update to update it right? I dong want backup.

For iOS7, you'll need the latest iTunes on your computer in order to activate. So start with updating that.

- next plug your iPhone into your computer running iTunes.
-iTunes will ask you if you want to update. When it does, it will ask you if you want to just download, rather than download install. Given the download speed and chance of dropped connection, you definitely want to download only.
- iOS7 image is > 1GB. At 15kbps, this would take more than 138 hours if everything went perfect. I believe the iTunes downloader can pick up where it left off, though am not sure. You may want to find a faster and more reliable download temporarily.
- once the image is downloaded, you can then do the upgrade. You'll need to be connected for activation, but bandwidth requirements are not great.
- while your computer is upgrading the firmware on your phone (phone is blank and has a white progress bar going across), it is very important that you don't sever the connection between computer and phone or else you may get the phone in an unrecoverable state. During the period, internet connectivity doesn't matter. Your computer should have a power source, as it's important your computer doesn't go down while the firmware is updating.

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so just to recap, the process is:

1) download os update (internet required, can be interrupted, possibly download resumed)

2) start firmware update (minimal internet required to start.computer to phone connection must not be severed during the ~20 minutes it takes to update, doesn't matter if internet goes out during)

3) activation (internet required, but bandwidth limit is small, interruption ok).