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Locate the folder where Lethal Company is downloaded. For most people this should be in: C:\Program Files (x86)\Steam\steamapps\common\Lethal Company (4).
If you are still unsure where the folder is, you can check in the Steam app > Librarby > Lethal Company > right click > Manage > Browse local files. This should open the folder where the game is.
Once you have located the game folder, drag and drop the downloaded files from the Lethal_Bigger_Lobby to the game folder. It may ask you if you want to replace several files, click accept to continue (5).