Windows – Access denied when modifing value of registry key [run as admin & owner of key]

Administrative commands need to run from an elevated command prompt. Standard command prompts in Windows Vista and newer don't run with administrator rights even if the user is an administrator.

Use REG ADD with the "/f" parameter to force overwrite.

REG ADD /? explains the parameters.

REG ADD KeyName [/v ValueName | /ve] [/t type] [/s Separator] [/d Data] [/f]

KeyName     [\\Machine\]FullKey

Machine     Name of remote machine - omitting defaults to the current machine 
            Only HKLM and HKU are available on remote machines
FullKey     ROOTKEY\SubKey ROOTKEY [ HKLM | HKCU | HKCR | HKU | HKCC ] SubKey 
            The full name of a registry key under the selected ROOTKEY
/v          The value name, under the selected Key, to add
/ve         adds an empty value name <no name> for the key
/t          RegKey data types
            [ REG_SZ | REG_MULTI_SZ | REG_DWORD_BIG_ENDIAN | REG_DWORD | 
              REG_BINARY | REG_DWORD_LITTLE_ENDIAN | REG_NONE | REG_EXPAND_SZ ]
            If omitted, REG_SZ is assumed
/s          Specify one character that you use as the separator in your data 
            string for REG_MULTI_SZ. If omitted, use "\0" as the separator
/d          The data to assign to the registry ValueName being added
/f          Force overwriting the existing registry entry without prompt

To insert a new value or toggle a string value from "false" to "true", use a command like this:

reg add HKCU\Software\Citrix\Receiver /t REG_SZ /v UpgradeDone /d true /f

Example which reads an existing value and appends some string before writing it back:

set append=XXXXXX
set key=HKCU\Software\myTest
set value=myValue
set oldVal=

for /F "skip=2 tokens=3" %%r in ('reg query %key% /v %value%') do set oldVal=%%r
echo previous=%oldVal%

set newVal=%oldVal%%append% 

reg add %key% /v %value% /d %newVal% /f

Note: This sample assumes that the old value and the new value have no blanks. Otherwise one has to add quotes and change the "tokens" parameter. Extra error checking omitted for brevity. However, all registry manipulating code definitely must do adequate error checking.