For your applications to stay working properly you need to keep your operating system as up to date as possible.
As you try to keep up your iPhone 3G or 3GS you are going to see that you need to update your iPod operating system to do things like updating your applications. Without this you are going to be stuck, and many of your programs that you rely on are going to end up not working. The easiest way to do this is by simply plugging into your computer and using iTunes for the update.
Connect to iTunes
Once you plug your iPhone into your computer using the proper iPhone or iPod adapter it will show up in the left panel of your iTunes. When you select it the middle section of the iTunes display will be dedicated to the iPhone, showing its technical specifications, media, and other things about the device. The opening, or first, tab is the “Summary" tab that has “stats" about your phone, the versions of the software on it, and options for dealing with it through iTunes. The middle version section is divided into two selections, the ability to update the software or to restore you iPhone if there is a problem. Click on the top update button.
Download the Update
An update window will come up asking you to agree to the standard Terms and Conditions policy. Once you do this iTunes begins to instantly download the operating system update. You can check this download status in the left pane in the “downloads" section right below the button that accesses the iTunes Store. When you select this it brings up a window in the central panel that shows what is downloading and how far along it is in a progress bar. Depending on your internet connection the download should only take a few minutes.
Once the download is complete iTunes will immediately begin “backing up" your iPhone, which will take another several minutes. From here it will update your iPhone, and now the applications that were denying their update should be able to update. Once it ihas completed updating, iTunes will give you a date as to when you should check for updates again (which is usually in a week). Keep your iPhone as up to date as you can and that way you can make sure that it works properly.