Tool: ILSpy (Free & Open Source) This is the industry standard. It converts the DLL back into readable C# code almost perfectly.
: Decompiling software may violate End User License Agreements (EULA).
Here is a comprehensive guide on how to decompile DLLs, focusing on the safest and most effective tools available (both desktop and legitimate web options).
JetBrains dotPeek is a powerful, free standalone tool that specializes in .NET assemblies. It can reliably decompile DLLs into C# and even export them as Visual Studio projects.