What kind of bird is this?
It's hard to tell at this age what it is because the first real feathers are just coming in and it's tough to get a good feel for coloration. If the nest was within reach putting it back into the nest when they are this size is a good idea. (The old tale of getting your scent on the bird and the parents won't accept it back is not right - most bird species don't have that good of a sense of smell). By now it's either found someplace to move to or it has expired so probably nothing we can do about it. If the bird has a few more flight feathers and is showing some good development a lot of times just move it to the nearest tree or shrub for protection.