Answers

Typically flat paint is not cleanable, unless your flat paint is Sherwin Williams Duration or Emerald in which case you can use some warm water and a sponge.

If it’s not one of those two products you really don’t have many good options.  You can try cleaning it with warm water and a soft sponge, but be very careful not to remove the paint from the wall.  You can also try touching up the dirty areas by feathering the paint in, and as a last resort you can repaint the wall.

Flat paint typically isn’t the most live able finish, but it does provide you with the richest appearance.  Sherwin Williams Duration or Emerald gives you the best of both worlds.  Duration is my companies go to interior wall paint, I highly recommend it.