Okay here is another similar method.

NO LOOKING Bjorn!!


;Declare variable and set options 
If Not @LogonMode
Break On
EndIf
Dim $SO
$SO=SetOption('Explicit','On')
$SO=SetOption('NoVarsInStrings','On')
$SO=SetOption('NoMacrosInStrings','On')
$SO=SetOption('WrapAtEOL','On')

;Declare our variables
Dim $Key, $Value, $Count, $i, $UpdateValue
;Assign read value from the Registry to our variable
$Key = 'HKCU\Software\KiXtart-Learning'
$Value = ReadValue($Key,'Round02')
;Check if there is an error reading the value, if so quit and set error
If @ERROR Exit @ERROR EndIf
;Declare an array to use
Dim $Binary[(Len($Value)/2)-1]
;Set our counter to 0
$Count = 0
;Step through the read value and create the array
;We take 2 steps so that each element will be part of the 2 character binary value
For $i = 1 To Len($Value) Step 2
;We now assign 2 characters each time through the array
$Binary[$Count] = SubStr($Value,$i,2)
;We reset the counter before going back through the process again
$Count = $Count + 1
Next
;Here we set the specific element to a value we want to change
$Binary[2] = "bc"
;Now we write that value change to the registry using a Join since the $Binary
;var is an array format.
$UpdateValue = WriteValue($Key,'Round02',Join($Binary,""),REG_BINARY)
;Check if there is an error writing the value, if so quit and set error
If @ERROR Exit @ERROR EndIf