A DLL is not (usually) "encrypted". It's compiled code, there's no decrypting it to get something you can read and comprehend.
You can TRY decompiling it BUT
it's very likely to break your license to use the product the DLL came with (if not, it'd have come with source code)
you'd be highly unlikely to get anything you (or most any "normal" human) would understand to the level it'd be useful knowledge
there are better ways to figure out what a DLL does, IF they have public access points and you have the tooling to read them (which most compiler toolkits that can generate software that can interact with DLLs will provide).
Are you asking about what functions and other things the DLL provides, then there are various tools that can display that information. A tool like Dependency Walker can help determining the exported functions and datatypes from a DLL.