Whenever we update our ios devices, we lose all our applications and when we reinstall them, we lose every whole thing we've done so far! Is there any solution for this?

Before you start the update copy all purchases (which will include apps) to you computer's iTunes via FIle > Transfer Purchases and then force a backup of the iPad by right-clicking the iPad 'Device' on the left-hand side of iTunes and selecting 'Backup'. A backup of a device doesn't contain the apps, just their content and settings, so for the update to be able to restore your apps (and therefore their content) back onto the device, it needs them in your computer's iTunes library