-in the link page you provided, it states that Secure Boot should be temporarily disabled. This was not mentioned or emphasized in the instructional video. So, does that apply to me and is it a must?
Yes you must disable secure boot else the computer will not boot from your USB device, please see my canned text below for "how to"
For computers that have UEFI as opposed to legacy BIOS, to be able to boot from your USB device you may need to disable secure boot and change UEFI to CSM Boot, not all computers and BIOS are the same, please refer to your user manual if you have one as the following steps are only one such example.
Restart the computer, Windows 8 and 8.1 from the Start or desktop screen move your mouse pointer over the upper or lower right corner of the screen, when the Windows Charms appear click the Settings Charm, click on Power and then the Restart option.
Windows 10, Click on Start,Power and then Restart.
While the computer is re-starting,you will need to continually tap or hold down the particular key that will allow you to access the BIOS on your computer, we will use the F2 key as an example here;
After restarting the computer, when the screen goes black, press and hold down the F2 key, wait for the BIOS to load.
Select Security -> Secure Boot and then Disabled.
Select Advanced -> System Configuration and then Boot Mode.
Change UEFI Boot to CSM Boot.
Save the changes and Exit the BIOS, commonly F10.
-in the video, it says the repair installation must not be done at boot in and only within Win10. when I checked with dell.com, it instructs to do it at boot in. so, how do I do it?
As you are using a USB device supplied direct to you by Dell you should follow their steps to the letter.
-is windows defender a 3rd party antivirus?
No, from Windows 8 onward`s the version of Defender that is an integral part of the OS itself offers the same real time protection as Microsoft Security Essentials, that is why there is no need to install any other AV and by not doing so you are removing any potential conflicts that can often occur when using a third party AV.
-will I be able to keep my files?
The tutorial that I post the link to shows you how to do a repair install of Windows 10 by performing an in-place upgrade without losing anything.
Right now, I’m waiting for a USB flash that I order to back up all my files (movies, programs..etc),
Good idea, as you are backing up all important data you may wish to consider trying a refresh of the present OS first, see below;
Refresh your PC Fixes software problems by reinstalling the OS while preserving the user data, user accounts, and important settings. All other preinstalled customizations are restored to their factory state. In Windows 10, this feature no longer preserves user-acquired Windows apps.
You are welcome