If you use Whatsapp to send messages to your friends and acquaintances, it may be worth knowing that system requirements will be raised in a few months.
As of October 24, it is namely iOS 12 or later that applies, which means that older models such as Iphone 5 and 5C are no longer supported.
If you have a newer model than that, you may need to update the system, something you do by opening the app Settings and go to General -> Software Update. Then just follow the instructions until the installation is complete.
If you prefer Android, version 4.1 or later applies.