Initial thing that you require to in such case is that to varify the system date. If you have not set the date properly then it will have issues while downloading the updates as it reads wrong date from system. If that is the case in your system then you can double click on the bottom right corner of the screen where you view the shown time. It will open the settings for the time. Set it to proper date and time and save it. After doing that you can again attempt to update it.