Q: I can't dwnlod os x5 for my ipad

If that's iOS 5 you want, does that mean your iPad has iOS 4 currently?  The latest version of iOS for iPads is iOS 6.

If you currently have iOS 4, you need to use iTunes on a computer to ugrade the OS on an iPad.  Starting with iOS 5, iPad (and other iOS devices) were able to perform iOS updates on the device itself.

This document has the procedure

http://support.apple.com/kb/ht4623 - See the section Update your device using iTunes.

You posted in the OS 10.3 (pre year 2000) forum instead of the iPad forum. To get answers to your question, next time post in the proper forum. See https://discussions.apple.com/index.jspa  I'll request that Apple relocate your post.

If you have an iPad 1, the max iOS is 5.1.1. For newer iPads, the current iOS is 6.1.3. The Settings>General>Software Update only appears if you have iOS 5.0 or higher currently installed.

iOS 5: Updating your device to iOS 5 or Later

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

How to install iOS 6

http://www.macworld.com/article/2010061/hands-on-with-ios-6-installation.html

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

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

If you are currently running an iOS lower than 5.0, connect the iPad to the computer, open iTunes. Then select the iPad under the Devices heading on the left, click on the Summary tab and then click on Check for Update.

Tip - If connected to your computer, you may need to disable your firewall and anitvirus software temporarily.  Then download and install the iOS update. Be sure and backup your iPad before the iOS update. After you update an iPad (except iPad 1) to iOS 6.x, the next update can be installed via wifi (i.e., not connected to your computer).

Tip 2 - If you're updating via wifi, place your iPad close to your router to preclude getting a corrupted download.

 Cheers, Tom