You can try the following first:
1. First make sure you cancel the upgrade!
2. Then press the power button and hold for about 5 seconds
3. Choose power off
4. Hold down VOL+ and VOL- and pres simultaneously the power button
5. Choose update but do not update... instead cancel the process!
6. Shut down the console and restart normally
However, when connected to the internet it will soon prompt for update.
If that doesn't work, you can try using the Reset Required Firmware Version from DevMenu. If those 2 options don't work, its possible something else was installed into your console when you installed a .NSP file that is causing your console to detect an available to update. However, we currently do not know what or where this file is so the only solution from this point would be to install the update(s).