Hey there! I absolutely understand how frustrating it can be when your device is having pop-ups like that. I would suggest clearing the cache partition of the device to see if that helps.
With the device off, press and hold the Volume Up and Power keys. When the device turns on and logo displays, release both keys. Use the Volume keys to highlight wipe cache partition, then press the Power key. Continue using the Volume and Power keys to confirm your choice if prompted.
If it is still telling you to update the device even after clearing the cache next I would suggest is to back up your device and complete a factory data reset just to see if anything needs to be pulled through from the update.
You can do that by going to and open Settings, and then tap General management. Swipe to and tap Reset, then tap Factory data reset, and then review the information. When you're ready, swipe to and tap Reset. If you have a security lock set, you will be asked to enter the credentials.
If neither of these are helping I would next suggest having a certified technician take a look at the device.
