Take the custom .upk file you downloaded and rename it exactly: Labs_Underpass_P.upk .
No, you can ignore the universal Mods.upk file for this method. Labs_Underpass_P.upk
Disclaimer: Modding Rocket League is done at your own risk. Always backup your files. Map files & packages - RL Modding Wiki Take the custom
For easier management, consider using the Rocket League Map Manager on GitHub to switch between custom maps automatically. Labs_Underpass_P.upk
Create a backup of the original Labs_Underpass_P.upk to avoid losing the default map.
Find your Rocket League map folder, usually located here: Documents\My Games\RocketLeague\TAGame\CookedPCConsole\mods