It's possible that your internet speed is too slow, and that's why it's constantly checking for an update.
Try resetting your iPhone: press and hold Home and Power button until you see the Apple logo. Let go of both buttons as soon as the logo appears. Your iPhone will power up on it's own. This kind of reset won't change settings or remove data. Maybe it will help with your problem.
There's a second option to update your iPhone, if over the air doesn't work: use iTunes for updating. iTunes will load the right software for your device, and then install it. Connect your iPhone to your computer, backup your iDevice (just in case), and then update it.