Choosing the entry other than the first system

It sounds like GRUB, the bootloader used by Ubuntu, is set to boot into Ubuntu by default without showing the boot menu. You can change this behavior by editing the GRUB configuration. Here's how you can do it:

Open a Terminal: You can do this by searching for "Terminal" in the applications menu or by pressing Ctrl+Alt+T.

Edit the GRUB Configuration: Run the following command to edit the GRUB configuration file:

sudo nano /etc/default/grub

Change the Default Boot Entry: Look for the line that starts with GRUB_DEFAULT=. By default, it's set to 0, which means the first entry. Change this value to the index of the Windows entry you want to boot by default. For example, if your Windows entry is the third entry in the GRUB menu, change it to 2 (remember, the index starts from 0).

Save and Exit: After making the change, save the file by pressing Ctrl+O, then press Enter. Exit the editor by pressing Ctrl+X.

Update GRUB: To apply the changes, run the following command:

sudo update-grub

Reboot: Restart your computer, and the GRUB menu should now show up with the selected default entry.

Thank you very much haryw.
Unfortunately, this solution does not work, as windows is broken.
As a result, starting Ubuntu is impossible and Ubuntu has to be reinstalled.
I have performed changes of the partition table with a partition manager on a Windows PE Stick.
On the HP support pages I have read, the HP notebook Bios ist broken and has be flashed.