Kernel panic after upgrade to 18.04, boot repair via usbflash failed

My boot directory seems to be full. I can not find any instructions on how to delete old kernels via usbflash. I do not know how to get to the boot folder of the disk in usbflash. And I'm afraid to delete reasonable data from my old installation. However, this should not be the case if I'm just manipulating the startup folder, right. By the way, it is also not possible to start from one of the other old installations listed in the Grub. I ran Boot Repair but it failed and returned this report to me:
The hints here ->
Kernel panic after the upgrade to 18.10
sounds good. However, I am overwhelmed with understanding the boot repair report and would be grateful for any tips on how to save my old data via usbflash or to run the old installation.

Many thanks for your help!