How to download iOS 5.1.1 software update from itunes without switch and plug to pc?

You need PC.

You can update your device wirelessly (available in ios 5 and later)

If not, you need a computer.

As part of the update and restore process, iTunes will (in order):

1. Back up key elements of your device, such as settings, Contacts, Calendars, Notes, and your Camera Roll. iTunes will not, however, back up media content from your iOS device, such as music, apps, books, movies, and TV shows.
2. Update your device to the latest version of iOS software.
3. Restore your device from your most recent backup.
4. Restart your device.
5. Sync back media content from your iTunes Library to your device.

If you are attempting to update using a Mac or PC with which you normally sync

If you are attempting to update using a Mac or PC with which you do not normally sync

Additional Information

Notes:

1. Completing your backup may take some time, depending on the amount of content on your iOS device.
2. If you are updating your iOS device on a computer with which you do not normally sync , or if you disconnect your device before the sync process is complete, you may notice that some media content that was previously on your device is no longer there. You can restore this content by syncing with the Mac or PC with which you normally sync.
3. Movie rentals purchased directly from your device will not get transferred to your computer when you sync and transfer your purchases.