How to update the iOS 8.1.1 to 8.4.1 in iPad2?

You can't, you can only update a device to the highest iOS version that it supports, which for an iPad 2 is the current version i.e. 9.0.2 - you can update to that via Settings > General > Software Update (if you do update then take a backup first, and preferably copy any purchases over to your computer's iTunes library via File > Devices > Transfer Purchases)