From there click through the folders portal > product firmware > Hikvision.
'04 Back Ends' is where you'll find the NVR firmware. Yours is an I series. To avoid potential issues you should update first to a 4.50.x, then a 4.51.x version before finally putting the latest version on. It's good practice to delete your browser history between each update (occasionally the update changes the directories so on reboot the menu page doesn't appear and deleting browser history resolves that)
'03 Front Ends' is where you'll find the camera firmware. It looks like all of your cameras are G1 platform models. Again you don't need to load all updates but you shouldn't jump a version where the middle number has changed. 002 is already on the latest firmware (5.6.6) Your last camera (006) can go directly from 5.6.3 to 5.6.6, but for 004 and 005 I'd maybe first update to 5.6.1 before 5.6.6.
It's good practice to delete your browser history between each update (occasionally the update changes the directories so on reboot the menu page doesn't appear and deleting browser history resolves that)
Yeah it doesn't happen too often, so as long as you remember that if you get any issues with the config page after it reboots - page not loading, strange looking fonts/styling that will sort it.
It did happen to when I was downgrading from V4.60 to 4.51 on an I-series NVR... I suspected deleting the browser history is what fixed it, but wasn't sure because I changed other things at the same time.