You can covert a dll into it’s IL using the ildasm.exe utility. This is usually installed in:
C:\Program Files\Microsoft Visual Studio .NET\FrameworkSDK\Bin
If you want to run this utility via command line it would help if you add the above path to your Environment Path variable.
Here is a sample command line that will disassemble a dll:
ildasm MyFile.exe /output:MyFile.il
You can reassemble the above il after making some minor changes via the ilasm utility. This is usually done to make some minor changes to an assembly like modifying the version no. of another assembly to which it links to.