For those who have fallen in love with 2016 Doom and are excited to see how things have changed in Doom: Eternal, you are going to have a few surprises. However, the appearance of the Doom Guy Armor update in the 2016 version resonated with many fans. It was a modern take on classic armor. For those who liked it, the skin is available in Doom: Eternal.
You can unlock it from the Fortress of Doom hub location. You have to find it though. It's on the second floor. From the main area on the upper location, go to the right down the stairs. Continue to the second floor, and from the main area of that floor go left again and you will find two doors that you need to go through. On the other side of the second door, there is a small open area with a bridge connecting to another place. There you can unlock the Praetor armor.
However, you must have two free Sentry Batteries to access it. You can acquire them from any mission anywhere in the game at any level. There is a hidden handle on some levels, but you can also earn them by completing all three challenges of any mission. Once you have your two spare batteries, head back to the unlock location to use them and you can now freely use the armor. To wear it you have to go back to the main menu of Doom: Eternal and choose it in the customization section for the Slayer.
There are a handful of customization skins you can wear and change in-game, like classic sea armor, which you can also unlock on the Fortress of Doom.