After you complete Doom: Eternal, the game is always available for you to play and you can run through it again at a higher difficulty or you can repeat missions to find secrets you may have missed. You don’t have to start Doom: Eternal over if you want to do that. All you have to do is proceed to the mission select option and choose what level you want to repeat.
The mission select screen is in an odd location. Rather than being in the main menu or being in the pause menu while you’re playing the game, you need to find it the Fortress of Doom. It’s on the massive ship that serves as your central hub that you frequently visit to unlock numerous pieces in the ship to expand your power.
To find it go to the front of your ship at your bridge. You need to stand at the center of this stage and then turn to your right. The mission select screen is right there, ready for you to choose any of the available missions for you to try again. Pick any of the missions you want to play again, and at the bottom, you can input any of the available cheat codes you’ve collected while playing the game. These cheat codes alter the mechanics of the game, giving you an array of abilities, such as providing you with infinite ammunition, infinite extra lives, instantly staggering demons, and much more.
If you want to increase the difficulty of these missions you need to go into your pause menu and adjust it in settings, and then go to gameplay to modify it.
You can repeat these missions how many times you want to unlock all of the secrets or kill all of the demons you find, over and over again.