On this page of the Doom Eternal guide, we will answer the question of how to change the appearance of the hero. We are talking only about cosmetic changes, that is, those that don't affect the skills of the hero or his effectiveness on the battlefield.
Important note: Doom Eternal doesn't have any microtransactions and all new skins for Doomguy be obtained by simply spending more time in the game.
As you progress through the campaign, you can improve the hero's armor, although improvements done to the Doomguy's armor don't affect his appearance. They are just various passive bonuses.
You can find new armors inside the Fortress of Doom - the hub visited in between the main missions. Among others, follow the narrow paths to the towers.
New outfits are initially located in rooms, which are impossible to get into.
You can solve this problem by using the Sentinel Batteries. You can find these batteries during exploration (they are one of the collectibles) and receive them by completing all the mission challenges.
Two batteries are required to open each room with new armor. You place them in slots to the left and right of the closed door.
When you unlock the entrance, you will receive a confirmation of getting a new outfit.
Another way to purchase outfits is just to make progress in the game. They can appear in the reward list after completing a mission or after being promoted to a higher rank (experience level). If you want to gain further ranks, online gameplay is required (even if you are currently playing a single player campaign).
New armors are available in the main menu. Select the Customize option from the menu, then go to Slayer and Skins. A list of unlocked outfits will appear.
You can see your new armor not only in the main menu but also in those cut-scenes where the FPP view is temporarily disabled to show Doomguy's model.