You would want to check to see if it's either screwed in or nailed in. If it's nailed in prying it off is the answer, but if it's screwed then you have to remove the screws.

As far as the composite tread, in theory, it's that simple, yes. Just be careful if you do have to pry it, composite material is notoriously soft and marks easily. Also, does the 4x4 post extend through the tread?

The stringers are probably bad. Agree. Once you have the bottom riser off you'll see if it's rotted. Wood with direct ground contact is almost always the first to show signs of deterioration. Depending on how the treads and risers were done you may have to pull the first tread also.

The spindles you should be able to cut off with an oscillating tool and replace with standard 1x1 spindle (at least that's what it looks like). Usually you need to drive a structural screw through the top and bottom rails to be sure it's solid. It's pertinent to check the top and bottom rails for signs of weakness and rotting (if so replacing the whole thing may be best).

The handrail unfortunately looks like it got massacred. You can try to bang in more nails and fill it with something but replacement is the better route there as well.