How to Read, Modify, Save specific line in a .txt file c# ?

Question

Hello there guys, sorry to bother you...
I got stuck on a small problem due to my little knowledge.
I have this file (program.txt)

:SHORTSOURCEMENU

REM -- MENU --

RAFAS

RAFASD

:SOURCESELECT

IF "%SOURCEDOT%" == "124" SET SOURCE=enUS

IF "%SOURCEDOT%" == "125" SET SOURCE=esMX

IF "%SOURCEDOT%" == "126" SET SOURCE=ptBR

IF "%SOURCEDOT%" == "127" SET SOURCE=enGB

I want lile 3 (rafas, to become):
set SOURCE=124

I want line 4 (rafasd, to become):

set SOURCEDOT=124

Now that the file has been written the way it will work, I need to rename program.txt to program.bat.

OR, I could CREATE a new file called program.bat instead of renaming, even better :D

Thanks guys