Firmware update for model D6220

I am an older guy (77) with very limited computer skills and I am having trouble installing a firmware update for my D6220 model router. This is an important security update and so far, I have managed to get all the ducks lined up to get the download process happening and I am careful not to touch anything during this process and it say's not to press reset or turn off the power during the process.  However, every time it gets to the end of the download it all closes down and says it has been reset (or similar) and that's the end.  The router then starts up again of it's own accord and away we go but no upgrade. Can you please help?