Sorry about that ; but that's very easy don't worry we ate here to help you out just stop forcing it to turn on rather look for simple tools to disassembled your device and after that you will be able to clean it all then after you device will work again Just follow this link iPhone 4S Teardown That will help you my dia
Did you download the update using the phone itself? If yes then try charging the phone above 50% then install the update as it tends to reject installation of updates at a low power percentage If that fails then try getting another package of the update. Hope that will be helpful
You mean the screws are not getting out? Or you have failed to see them? If they can't be unscrewed using that bit Philips try a bigger bit or try to hot the screws using the soldering iron then unsrew them immediately.