PhD.-Application; How to deal with reference-letter requests when being out of academia for 2 years? Two years ago I did a M.Sc. in cognitive Neuroscience and then started working as a DL / ML-Engineer up until now. Back then I got two positive reference letters for a specific graduate program from my supervisors. 

I want to do a PhD now, but I wondering how I should deal with reference letter requests. In my current company there is no one with technical expertise in Data-Science or Machine-Learning whatsoever.

I see two options now: 

a) Should I ask my old supervisors to update my reference letters?
b) Should I just send the old ones, maybe adding some short note why the reference letters are old and obviously for some other program? 

Any recommendations on what to do?
I suggest asking your old writers or others at the institution for new letters. Treat it as a chance to catch up with old professors which is a good thing to do for any prospective academic. 

The old letters have less relevance and new ones, while still dealing mostly with what you were like two years ago can give some assurance that you've been active in the interim. 

Two years isn't that long. But if the gap gets much wider people will start to forget you. This is a good time to renew old ties.