I have an old iPhone and I don t have button Software update at general settings and I don t download apps but The update is requis ios 4.5

Note:

The iPhone 3G can only update as far as iOS 4.2.1

Connect to iTunes on the computer you usually Sync with and “ Check for Updates “...

If an Update Appears Install it... if not... you are up to date for your particular Device...

See the Using iTunes Section Here...

How to update your iPhone, iPad, or iPod touch

To update iOS versions prior to iOS 5 you need to use iTunes on your computer. Start iTunes on the computer you normally sync with. You might as well use the menu to update iTunes on your computer to the latest version.

Start iTunes on the computer and connect your iPhone. Select your iPhone in iTunes and select Summary from the row of choices. Scroll down to find Check for Updates.

Note: your iOS version will be updated to the most recent version supported by your phone.