Ask HN: How can I take charge of how software should work?

For 1.5 years I’ve been assisting in the maintenance and enhancement of software that was built by a consultancy almost 3 years ago. No one on my team was part of the initial design, build, and launch. The documentation for the software is lacking and there is no one who I can go to to get confident feedback on how features should work.

Part of me want to leave the job cause it seems like a hopeless situation, but the pay is great. I want to turn the project around but that seems impossible cause there doesn’t appear to be a “true north”.

What can I do?