Comment by cloudking

This is not just first day advice, but more general advice for new PMs:

Talk to your users relentlessly, find out how they use and don't use your product. Get a deep understanding of their workflows and user journeys in the product.

Trim the fat (shift focus) and solve problems they have that the product doesn't solve yet or solve well.

Reduce the steps in their critical user journeys. For example, if it's something they do every day, going from 5 clicks to 3 clicks adds up over time and improves satisfaction.

Dive into metrics and implement quantitative metrics where they don't exist. Survey users for qualitative metrics.

Bring data (metrics, market research, customer quotes etc) to executive meetings to back up your ideas, data speaks louder than your words.

Basically, if your product is in the market you don't need to always guess what to build, your users will guide you. That's not to say you can't innovate too, but a large part of being a PM is bringing the user experience and their frustrations to your team to action.