I believe you can do this with ILDasm or .NET Reflector.
Open a Visual Studio Command Prompt ... Launch ILDasm
If you open the manifest, you can get the .ver version, and the mscorlib version should be the target version. E.g. the following dll uses 4.0
// : v4.0.30319
. extern mscorlib
{.publickeytoken = (B7 7A 5C 56 19 34 E0 89)//.z\V.4...ver 4: 0: 0: 0
}