The first thing you need to know is that Lean and Six Sigma are two separate approaches to achieve the same thing.
Lean focuses on the elimination of wasteful steps in a process, whereas Six Sigma targets a reduction in process variation.
For a number of years people argued the superiority of both approaches and typically you were either in one camp or the other, a bit like Apple or Microsoft.
But people began to realise pretty quickly that they could be used together as a powerful business transformational tool and Lean Six Sigma was born.
The second thing is regarding accreditation. Nobody officially owns Lean or Six Sigma and therefore there is no official accreditation or BOK (Body of Knowledge. Some companies have been setup to provide this service and set a standard but it is their own standard. Six Sigma is a registered trademark of Motorola but they do not provide accreditation for it.
The most important thing is for you understand it, apply it and practice continuous improvement in your organisation rather than have a certificate which says you have passed an exam.