Normally, the only way to install or restore your iDevices IPSW file (IPSW stand for iPhone SoftWare) is to connect your iPhone to your computer, start iTunes, and see if there is a new version of iOS available, or restore a backup of the current version.
However, besides jailbreak programs, there is another way to install an IPSW file on your iPhone!
There are various sources on the internet where you can download every version of iOS that you want (try here), and after that, you can simply install the newly obtained file via these simple steps:
1. Make sure you have the most up-to-date version of iTunes
2. Launch iTunes
3. Connect your iDevice to your computer
4. Select your device in the list on the left side bar of iTunes
5. Select the “Summary” tab in iTunes
6. Hold the Alt-key (Mac)/Shift-key (Win) and click on “Restore”
7. Instead of just restoring, iTunes will show you a dialog box to ask you for the location of the IPSW file that you want to use for restoring your iDevice
8. Select the file you just downloaded and hit the “Choose” button. Beware, some browsers may save the file as a “.zip” file. If this happens, just rename the file, and change the extension to “.ipsw”. This will make the file selectable again in iTunes.
9. Wait for iTunes to restore your device, go drink a coffee, and prepare to enjoy your newly installed iDevice!
IMPORTANTwe do not encourage downloading IPSW files from the internet, and installing them yourself. The only legit source of IPSW files is Apple, via iTunes.
Then why would you want to use this page? Often, a new version of iOS can bring you nifty new features, but on older devices, this comes with the cost of speed and usability. Apple offers no option to uninstall an iOS update, or to simply downgrade your iPhone. Therefor, this is the only option you have.