You’re on a team with people that have different titles and roles: product manager, product owner, your boss, principle software engineer, etc. It’s always good in any context to know, when priorities conflict or trade-offs have to be weighed, who the person is that you go to to break that tie. Sometimes it will be your boss; sometimes it’ll be the product manager; sometimes the principle software engineer. The projects you work in intersect in different ways with these and other roles. Understanding those roles will make your job easier which will have a direct impact on your team's productivity.
Think of your tie-breakers as allies, too. They can help you do more and move quickly by removing ambiguity and road blocks. But it's critical that you don't just present problems to be solved to these allies. Your job isn't to make work for others - it's to help reach your organizational goals and create value. This means when you encounter a situation where you need a tie-breaker of any kind, it's up to you to do your homework first. Make sure you can present reasonable options and have thought about the trade-offs. That will help your tie-breaker make an informed decision and feel comfortable that they're not taking on risk by helping you out.
Once you've got your answer, make sure you document it somehow. Maybe it's an email summarizing the decision; maybe it's updating documentation; perhaps it's adding stories to your backlog. The mechanics of it don't really matter as much as documenting it and communicating it to your team and stakeholders. This will help prevent second guessing later on as well as reinforcing your reputation for transparency.
Remember, too, that for some people you are the tie-breaker. When you're the tie-breaker, work with those that are coming to you so that the topics I cover above are addressed. Being decisive is all well and good, but if you make a poor decision because you didn't have all the facts, that's on you.
But what happens if you don't have a tie-breaker? What if things are so changeable or chaotic that you don't have a person to go to? This is definitely a tough spot to be in. My recommendation is to first, start building relationships with people that could become your tie-breakers. This will serve you well in many situations, not just resolving conflicts. Next, transparency becomes even more important if this is the case you're confronting. By keeping people informed, in whatever way is appropriate in your organization, you'll build a reputation for level-headedness and communication. You'll also likely learn whom your tie-breakers are as you get feedback about your decisions. They may not be "official" tie-breakers, but they're a start. Build out relationships with these people.
Knowing how to break ties when it comes to decisions of all kinds in your team and organization will help you become a better manager and teammate. It will help build and reinforce solid relationships that will help you in all aspects of your job.