In Starfield, you'll get to complete various challenges and quests, one one which is the Keeping the Peace mission. You can start this mission early on after reaching New Atlantis by visiting the United Colonies Security Office. Once it begins, you'll retrieve a package for UC Security, which of course you'll need to know how to find. Here's a complete walkthrough for the Keeping the Peace mission in Starfield.
Starfield Keeping the Peace mission guide
Keeping the Peace is a side mission you can start by visiting the UC Security Office after landing on New Atlantis (planet Jemison)—it's the first building you'll see after crossing the inspection area.
Once inside, speak to Sergeant Yumi at the office, and choose the dialogue option that asks about applying for jobs. Yumi will task you with retrieving a package for Agent Plato—one of the seniors at Aegis.
Where to find Agent Plato in Starfield
After taking the mission, visit the MAST building. Take the elevator inside the building and go to the Defense Research / Health and Human / Aegis floor, where Agent Plato is located. He informs you about the location of this package; you'll need to return to the spaceport area to collect.
Where to find Agent Plato's package in Starfield
Agent Plato's Package is located under a bench at the GalBank ATM in the Spaceport area on New Atlantis.
Check under the bench, collect the package, and return it to Sergeant Yumi.
Starfield Keeping the Peace mission rewards
You'll be rewarded with 100 XP and 2,000 credits for completing the Keeping the Peace side mission in Starfield, and be set up for more missions from Sergeant Yumi. I recommend spending your new credits on essential resources, as there are a lot of crafting and research options in the game.
Want to read more about Starfield? If so, check out Where to get Polymer in Starfield? on Pro Game Guides.