I cannot use my cellular phone because I get the message that an updated is needed I do not have know how to get the update

My settings say I have a cellular error and my phone needs an update yet I have not received any other notifications on how to update. I left it plugged in overnight which is typically how the updates are download it and that did not work I can get into the Internet with Wi-FiBut phone applications are not working I am not very well versed on these type of matters