Labs_underpass_p.upk • Real

Open the game, select "Freeplay" (or local match) and choose "Underpass". Your custom map will now load. Pro Tips for Map Modding

Create a backup of the original Labs_Underpass_P.upk to avoid losing the default map. Labs_Underpass_P.upk

This method is primarily for local play and Freeplay. Open the game, select "Freeplay" (or local match)

You don’t need to swap files between playing online and playing offline. Open the game

Disclaimer: Modding Rocket League is done at your own risk. Always backup your files. Map files & packages - RL Modding Wiki