Metal Gear Solid V doesn't offer any option to change difficulty level. However, if you have some trouble with remaining undetected, you can use a "legal trick" which will make it easier. In the main menu, go to options and search the tab on which you can activate the Chicken Hat. Once activated, you can start the game. The Chicken Hat will become active when you will die during a mission and return to a checkpoint. Thanks to wearing it, enemies will have much more trouble with finding you, even if you're in their range of sight.
Important! Using the hat is limited - you can use it only three times to deceive enemies. Additionally, using the hat blocks the ability to receive S rank in missions (the highest possible rank to obtain with the hat is A).