Once the update is applied, you will automatically be brought to the main menu. After doing so, you will be met with a loading screen during which the update will apply. Generally, playing a couple of games in 'Play Now' mode is enough to trigger the download. When your game is completed, NBA 2K18 should prompt the update message in the corner which requires backing out to the main menu.
After that, you should be able to play without a problem. That said: Once the download is complete, you'll see a pop-up message asking you to return to the game's main menu so the update can go into effect. Exactly how long this takes varies, depending on the size of the download and the speed of your internet connection. If that's the case, you'll have to wait for the data to download to your console or PC. When that happens, it means you can't continue on to play until you've got the most recent update downloaded. If you're trying to play NBA 2K18 but are receiving a message with the error codes 'EFEAB30C' or '4B538E50', a common reason for seeing these errors is because you don't have the latest data downloaded.
