Editing and saving iterations .csv files

I am trying to solve an optimization problem. This involves reading in a input.csv input file to another program that goes through some calculations. This input file consists of a few rows of headers (strings and numbers), followed by the data. The data is organized into 4 columns: t, x, y, z. For my purposes, I needed to

1) Read in the z column from the orginal .csv input file

2) Iterate on those values.

3) Edit the original file, and replace the z column.

4) Save that new edited version of the edited_input.csv file, with a different (iterating) file name. In order for this new edited_input.csv file to work , it needs to contain all the original headers.

I've figured out steps 1 & 2. I am able to read in the data values from the original .csv file. I have edited the Z column, and saved that array as a new variable. The difficulty I am having is steps 3 & 4. Can someone please point me into the right direction?