To change your difficulty, click the little chat button next to you chat box. Go to Set Notoriety. There you can set the level of mobs compared to your own (mobs will usually be anywhere from at your level to one over at the “0” level difficulty. Go to -1 means mobs will be one level below to the same level difficulty). Remember bosses at your level will show orange.
You can also change how many mobs you face. At x2, you will face mobs as if you were a team of 2. You can choose if you want bosses to spawn in missions as well. Some missions will have a boss no matter what. Later on in the game you will some Arch Villains in your missions. The last option on Set Notoriety allows you to scale those down to Elite Bosses, which are harder than regular bosses, but easier than Arch Villains.
If you want to change these settings after starting a mission, then you must go to the mission exit and leave the mission. Then make your changes and select a different mission. This will reset the mission, but at the new difficulty.
Hope this helps, don’t be afraid to ask more questions