Did mistake of doing PhD in a dead field. How to approach postdoc applications with hope of changing fields? I am a final year PhD in a STEM field. My PhD research field is fairly stagnant with less scope of future growth. I had chosen this topic because I was interested in the field during my masters and did not investigate seriously about the future prospects in the field. 

Now I totally regret it. I managed to publish just 1 publication during my 5 years of PhD, and am planning to submit 2 more. There was no scope getting an internship in my research area and while searching for industrial positions, I have found that such positions are also nonexistent. Lot of red flags for a research topic.

I wish I could have been more serious while investigating for PhD positions. I could have done many things differently. I want to be in academia. And I am looking to switching to a different field during my postdoc to make my profile better. How should I approach a prospective professor if I have nothing to show competency in the new field? Is it even possible, if there's an overlap of just a simulation technique between my work during PhD and the field I want to work in?
To some extent, it's expected that you're not an expert in the field of the post-doc. One of the points of doing a post-doc in the first place is to broaden your research profile.

The thing you should be worried about is your track record - from the prospective advisor's side, one of the most important questions is whether you're able to effectively finish up a research project. If one finished publication is way beyond in the norm in your field, then you should indeed be worried; if it is the norm you may be fine.

The other aspect is to put up a convincing job letter that will explain why you are the right fit -- you may want to emphasize relevant skills and experiences and explain how they will help in the job you apply for.