Users with old model iphone device such as iphone 3 may encounter problem while installing apps such as facebook on their mobile devices, this is as a result of running on low ios.
Today with the help of Whited00r such users can upgrade their phones to ios7.
Whited00r supports iPhone 3G and 2G along with the first generation iPods touch. Whited00r is a great program that supports ARMv6 versions of OS like it was with iOS 6 on iPhone 3G / 2G. This program will not affect your device in any negative way.

Step 1. Backup your iPhone 3GS and remember that you are installing the tool at your own risk.
Step 2. Download Whited00r7 Program for Windows or Mac and your iPhone / iPod touch model. Download the program here.
Step 3. You will have a zipped archive with the program and all the tools necessary for the installation process. Unzip the folder.
Step 4. Launch Redsn0w program. You need to select “Extra” option and choose “Pwned DFU” button. The software will show you the guide so, please, follow it and wait till your smartphone is set into Pwned DFU mode [you will see the black screen at this point].
Step 5. It’s time to install Whited00r7 program with iTunes. To do this, you should press / hold SHIFT on Windows PC and point to Whited00r .ipsw file located on your computer.
Step 6. Finish the installation procedure and reboot the iPhone 3GS.
Source: www.kaesynwosu.com/2016/03/how-to-upgrade-to-ios-7-on-iphone-3gs.html