My IPhone does not have software update button.....

You must upgrade through iTunes. That feature requires iOS 5 or later which you apparently don't have. So see:

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

http://support.apple.com/kb/HT4972

Read the article you linked:

iOS: How to update your iPhone, iPad, or iPod touch

It tells you how to update via itunes.

You must read the entire article, not just the first part