How do you re-position a product if you haven't reached product market fit?

I think this really depends an enormous amount on why you haven't reached product fit. So to reposition, you need to continue to iterate: your current GTM plan and positioning should lead to additional research, drawing conclusions, and then rephrasing and updating value prop, messaging, etc., GTM again, A/B testing, analyze results and repeat. Research can include a lot of reading, but also analysis of your data (assuming you're already out of stealth), as well as user research - interviewing (not only your users but users who have never seen your product), tracking usage and so forth.

If you haven't reached product market fit, then you don't have much to protect, to risk. That's a great position to experiment with your positioning :)
Think of it as if you're searching to see what works - search in the problem and customer space. This means you need to figure out who you are going to solve and what problem you're going to solve.
At this point, the question is still to open to give a precise and prescriptive answer.