There is a CSV reader/writer in python that you can use. CSV files don't really have cells, so I will assume that when you say "B1" you mean "first value in second line". Mind you, that files do not behave the way a spreadsheet behaves. In particular, if you just start writing in the middle of the file, you will write over the content at the point where you are writing. Most of the time, you want to read the entire file, make the changes you want and write it back.
import csv
# read in the data from file
data = [line for line in csv.reader(open('yourfile.csv'))]
# manipulate first field in second line
data[1][0] = 'whatever new value you want to put here'
# write the file back
csv.writer(open('yourfile.csv', 'w')).writerows(data)
B1but csv files are not addressable as rows/colums. You can create a list of lists (basically a list of rows) do the update and then save.