Can i manually decrypt a folder encrypted with ES File Explorer? I had encrypted a folder using ES File explorer. Then later on, I upgraded my ROM from cm12 to cm13. And then, I was not able to decrypt the data with ES file explorer, after the OS update. 

I know the key used for encryption. Is it possible to decrypt the data manually?

I thought it was not possible, but then i saw this post How to decrypt Adopted Storage? And now i believe there is a solution for my problem too.
Try this tutorial link - https://www.quora.com/How-can-I-decrypt-an-encrypted-ES-explorer-file-without-the-password

And this - http://blogs.candoerz.com/question/159398/es-file-explorer-forgotten-password.aspx

Hope I helped,

Sid