To access your windows registry in startup repair, you need to load it first in a temporary key then run regedit.
Suppose you want to edit the HKLM\Software key, load it with the following command
REG LOAD HKLM\Tmp D:\Windows\System32\config\Software
Suppose you want to edit the HKLM\Software key, load it with the following command
REG LOAD HKLM\Tmp D:\Windows\System32\config\Software