how can i start to upgrade my iphone software from 4.2.1 to latest software

Your iPad cannot be upgraded. See:

See the chart below to determine whether you can upgrade your device and what you can upgrade to. If you do not have a Software Update option present on your iDevice, then you are trying to upgrade to iOS 5 or higher. You will have to connect your device to your computer and open iTunes in order to upgrade.

IPhone, iPod Touch, and iPad iOS Compatibility Chart

Device iOS Verson

iPhone 1 iOS 3.1.3

iPhone 3G iOS 4.2.1

iPhone 3GS iOS 6.1.x

iPhone 4 iOS 6.1.x

iPhone 4S iOS 6.1.x

iPhone 5 iOS 6.1.x

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iPod Touch 1 iOS 3.1.3

iPod Touch 2 iOS 4.2.1

iPod Touch 3 iOS 5.1.1

iPod Touch 4 iOS 6.1.x

iPod Touch 5 iOS 6.1.x

-------------------------------------------------------

iPad 1 iOS 5.1.1

iPad 2 iOS 6.1.x

iPad 3 iOS 6.1.x

iPad 4 iOS 6.1.x

iPad Mini iOS 6.1.x

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Select the method most appropriate for your situation.

Upgrading iOS

1. How to update your iPhone, iPad, or iPod Touch

2. iPhone Support

3. iPod Touch Support

4. iPad Support

a. Updating Your iOS to Version 6.0.x from iOS 5

Tap Settings > General > Software Update

If an update is available there will be an active Update button. If you are current,

then you will see a gray screen with a message saying your are up to date.

b. If you are still using iOS 4 — Updating your device to iOS 5 or later.

c. Resolving update problems

1. iOS - Unable to update or restore

2. iOS- Resolving update and restore alert messages

If you have iPhone 3G, IOS 4.2.1 is the latest and greatest, there's no more update for that iPhone.

Type in your serial number (from iPhone Settings > General > About > Serial Number) to find out which model you have:

https://selfsolve.apple.com/agreementWarrantyDynamic.do

What device do you have?

You will need to connect to a PC/Mac that has iTunes to update the iOS. The article below can show you how:

http://support.apple.com/kb/HT4623