I totally screwed up the update from openSUSE Leap 15.3 to 15.4 by my own fault. Now on boot Window 7 is shown as the only alternative.
I want to create a bootable usb storage device to see if I could
fix things.
Luckily, I do have a second computer running kubuntu. Using it I
did the following
I downloaded
openSUSE-Leap-15.3-KDE-Live-aarch64-Build10.25-Media.iso
I formatted my usb storage device
mkfs.ext3 /dev/sdb1
and used dd to copy the iso-file onto
the usb device.
I started the openSUSE computer with the usb device plucked in,
hit ENTER and F12 and arrived at a selection between four boot options.
I chose USB HDD: HTS54804 0M9AT00
but nothing happened.
What is wrong here? Is something wrong with the usb storage device?
Is the openSuse computer in a state that it can't read it?
Please, help me!

You may need to use this iso:
openSUSE-Leap-15.3-KDE-Live-x86_64-Build10.25-Media.iso
the iso you're using is for an arm system.

Using which upgrade method? Live/online using zypper? 15.4 installation media? If the latter, it can do the repair. It can be used to boot the installed system too.
Good plan if you did a live upgrade, Leap is your only Linux OS installed on your PC, and you don't already have correct installation media.
Why? You're wearing out your USB device by formatting it, then overwriting the fresh format by dding an .iso onto it. Formatting is what you do so that you can copy things on and off of it. An .iso is a file full of files and formatted filesystems to be writing directly to a device, not to a (formatted) filesystem.
Is that an external HD? A USB stick? The disk you installed Leap on?
Probably what colorpurple wrote. Windows PCs don't use aarch64 operating systems. You need an x86_64 .iso for all the Windows PCs I'm aware of.

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Here are my answers to mrmazda's questions

I was using zypper for online upgrade. Set the releasevar parameter to 15.4 and started
sudo zypper dup. My mistake was not to take the advice serious to do this in non-GUI mode.
Everything went well for a while. Almost at the end of installing the downloaded packages the
screen went blank and showed a garbeled error message. I could revert to CLI in non-GUI mode but resuming
"dup" was blocked. When I turned of the computer and started it again the only boot options where
the Windows 7 OS which had been there as a dual boot all the time.

The usb device I am using is an external hard disc.

I will now download the iso version you colorpurple suggested and not do reformatting of the usb HD but
use dd right away. I'll keep you posted.

I am back again.
I did as I mentioned in my last post:

downloaded openSUSE-Leap-15.3-KDE-Live-x86_64-Build10.25-Media.iso
then
dd if="file above" of=/dev/sdb1

I checked the result with my kubunto machine and it looked ok.
Started the first computer with it but chosing the HD boot option produced no result.

How is it shown? Is there a Grub menu that only shows one item, which is Windows? If so, you should be able to exit the menu to the grub shell and boot manually, with instruction. To help, we'd need to see /etc/fstab from the installed system, and output from grep -A5 openSUSE /mnt/boot/grub2/grub.cfg (after mounting Leap's root filesystem to /mnt), lsblk -f & fdisk -l retrieved via booting any Linux live media. There are howtos for booting from the Grub shell scattered about the net, maybe here somewhere.

It's weakly possible all you really need to do is use your PC's BBS menu to select what to boot from, if it's a UEFI PC, and installation was done in UEFI mode. Whether it's a UEFI installation on GPT should be evident to us from fdisk -l output.

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Wrong. Try again using /dev/sdb instead of /dev/sdb1. .isos must be dd'd to disks, not to partitions.

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Thanks!!
that worked.
Now it remains to see, if i can save the previous system.
But that is another issue.