How can i update my iPhone 3G to iOS 7 because i wanna download Facebook app. but it requires iOS 7 or more.

Hi Karin, to best of my knowledge you cannot update a 3G much beyond iOS 4.2.1 or so. I think even the 4G does not allow newest iOS 8.1.2 - the 4S does.
Even if you mean Mac OS the newest version is 10.10 - 11.1 may be in beta, but not anything that would effect your 3G.

Good luck!

(Just to be clear, this is the BBC Radio app, right? If your problem is the BBC News app, I may have a different suggestion.)

Do you have the latest version of the app? It appears that BBC Radio just updated the app a week ago on 31July2014 (Version 1.5.3), but it requires iOS 6.0 or later and is optimized for iPhone 5. I see you have an iPhone 4.

Do you have iOS 6 on your phone? And do you have the latest version of the app?

iOS 6 is supposedly completely compatible with iPhone 4s, and it is also supposedly compatible with iPhone 4 except iOS 6 on:

• iPhone 4, does not support:
  • Siri
  • Maps flyover
  • Turn-by-turn navigation
  • FaceTime on 3G
  • Hearing aid support

So, it seems to me that this app should still work on your phone, unless you don't have the proper operating system or you haven't upgraded to the new version.

(the info below is copied and pasted for your viewing)
What's New in Version 1.5.3 FIXED - problem with automatically finding your local station
Please note that you can also choose any local station from the option beneath the dial after turning it to 'Find Local Radio'. You can even add up to 3 selected local stations to the dial using the Edit function.
Compatibility: Requires iOS 6.0 or later. Compatible with iPhone, iPad, and iPod touch. This app is optimized for iPhone 5.

delete the old facebook app now and install again the new version from the app store
http://www.theverge.com/2013/9/18/4744904/how-facebook-secretly-redesigned-its-iphone-app-with-your-help
Facebook iOS 7 iPhone App Update Is Here Flat UI New Navigation Bar And...

to update to iOS 5 is only possible on iPhone 3GS / 4 / 4S with connecting the device to iTunes on a computer.
- Plug the iPhone to your computer and open up iTunes under devices click on your iPhone and then click on check for update.
----
latest iOS possible:
iPhone 1 = iOS 3.1.3
iPhone 3 = iOS 4.2
iPhone 3GS = iOS 6.1.6
iPhone 4 = iOS 7.1.2
iPhone 4S = iOS 8.1.2
iPhone 5 = iOS 8.1.2

iOS is not an app - it is the running system of an iPad and is to be dowloaded from Apple:

- the download depends on the model of your iPad:
For the iPad first generation (A1219) to update is only possible with connecting the device to iTunes on a computer.
- but this model supports only iOS 5
- NO iOS 6/7/8
-------------------
for all other models:
There are two ways to upgrade the iOS version of your iPhone, iPad, or iPod Touch - via connecting the device to iTunes on a computer
- or directly from within the device, but only snce iOS 5, it is possible to upgrade the iOS version without the need to connect the iOS device to a computer (over-the-air update)
just go to: settings - general - software update

if you want to update your phone and require the iphone firmware
you can just connect your phone to itunes and update from there
if you require a specific firmware try http://www.ipswdownloader.com/
if you are looking for ios 6 for your iPhone 3G i have to tell you that iphone 3G only supports firmwares up to iOS 4.2.1

If your phone is out of space you wont be able to download the app, to fix this delete some apps or music. Also you need an apple account to download apps. Another problem you may have is that the iphone doesn't have the most up to date ios, if it is an iphone 3s or older it cant update to ios 7 and you need ios 7 for many apps.

Unfortunately, that will not be possible. Apps are written to be compatible with the most current hardware software profile. iPhone 3G highest iOS revision was iOS 4. There have been 3 revisions to iOS since then, and 3 upgrades to the iPhone hardware. The App Store only provides support for compatible hardware software profiles. Legacy software is not available. Sorry for the bad news. Good luck.

Because Whatsapp has updated past the IOS your phone has, it cannot be downloaded to the iPhone 3G. In order to get Whatsapp on your phone you'll need to upgrade it!

In your Settings App, go to "General" then go to "Software Update". From there you can upgrade your software without using your computer.