Windows 10 Backup.

To make a backup of how the entire system is at any given point in time, you require a system image
in other words an exact image of everything on the computer
That will not fit on most flash pens and even if it would it is an unsuitable way of storing an image
You need the image on an external hard drive

You either recover the image from the advanced trouble shooting menu
OR you recover the image from a flash pen made in windows 10 - recovery drive
OR you do so from the installation media.
Whichever path you use, the drive with the image must be attached before you start

I will guide you further when you reply

I would not store it on a flash but you go to Control Panel - File History and then lower left corner of window - system image.
You can only image drives formatted NTFS but they can be UEFI and have GPT partitioning OR they can be MBR with the traditional system BIOS

After you have the image, you invoke recovery from the image either by booting from the install media and then follow this
To restore your computer from backup, connect the drive with the system image backup and reboot your computer with the Windows installation media. During the Windows Setup, click Next, then click the Repair your computer link in the bottom-left corner of the screen. Click on Troubleshoot, click Advanced options, and select System Image Recovery. Now select the target operating system you want to recover, click Next and Finish.

OR if you can get to recovery options from power button on lock screen or into windows, you can of course access the advanced menu and then recovery from there.

Or you can boot from a windows 10 recovery drive without copying to that system files.
THAT is NOT the system image drive
The recovery drive is made on a smaller usb pen and allows you to boot to the advanced start options and from there to troubleshoot etc and then to system image recovery
http://www.tenforums.com/tutorials/4200-recovery-drive-create-windows-10-a.html

Little point in actually making that - if you can definitely boot from the install media.
However on an old computer if that install is on usb, the computer must be capable of booting from usb of course.

Hope you do not mind me telling you but it is vital that test both the recovery drive and check that it then finds the image albeit there is of course generally no need to actually process it to completion.

Also please do not forget that an image is only going to restore as at the date of the image so additions and changes etc will not be there
It is of course therefore vital that you keep copies of important personal data either by simple copy and paste or file history.
Remake the image as and when you see fit, but not of course merely to add a few docs pics and music etc.