Create a csv file with python and put in nested list to it

To be able to update existing values (lets say based on the bike number), you would first need to read your file in and store the data in a dictionary. You could then update the dictionary entries based on your new data. Now you can sort the keys in the dictionary (if you want the file to remain in a sorted order) and write the updated data back to your file. For example:

import csv

filename = 'Bike_List_With_Service.csv'
header = ['Bike No.', 'Purchase Date', 'Batt %', 'Last Maintenance', 'KM since Last', 'Service?']
existing_data = {}

new_data = [
    ['T101', '10/4/2016', '55', '10/1/2017', '25.08', 'N'], 
    ['T102', '1/7/2016', '10', '15/5/2017', '30.94', 'N'], 
    ['T103', '15/11/2016', '94', '13/6/2017', '83.16', 'Y'], 
    ['T104', '25/4/2017', '58', '10/1/2017', '25.08', 'N'], 
    ['T105', '24/5/2017', '5', '20/6/2017', '93.8', 'Y']]

# try and read in existing data from the file
try:
    with open(filename, 'r', newline='') as f_input:
        csv_input = csv.reader(f_input)
        next(csv_input) # skip over existing header

        for row in csv_input:
            existing_data[row[0]] = row[1:]
except FileNotFoundErrors as e:
    pass

# update the existing data with new_data
for row in new_data:
    existing_data[row[0]] = row[1:]

# write the updated data back to the csv file    
with open(filename, 'w', newline='') as f_output:
    csv_output = csv.writer(f_output)
    csv_output.writerow(header)

    for row in sorted(existing_data):
        csv_output.writerow([row] + existing_data[row])

This will first try and read an existing file (if the file does not exist it will continue). The dictionary key holds the bike number and its value contains the remaining row values.

By using Python's csv library, it takes care of reading a row of comma separated values and converting them automatically into a list per row for you. If the values had strings in them with quotes, it would deal with that too. If you are don't wish to use a library, the alternative is to read each line of the line in, remove the newline at the end and use split() to create your list.

import os
        if (not os.path.isfile("/{}/Bike_List_With_Service.csv".format(filepath))):#checks if Bike_List_With_Service.csv is in path
            with open(filepath + 'Bike_List_With_Service.csv','w',newline='') as file:#creates a new csv file named 'Bikelistwithservice.csv'
                a=csv.writer(file,delimiter=',')
                data=data
                a.writerows(data)#write each line of data
        else:
            file_to_be_overwrite="Bike_List_With_Service.csv"
            ov=open(filepath+file_to_be_overwrite,"w")
            ov.truncate()       #clears everything in the file and reenter the data
            ov.close()
            with open(filepath + 'Bike_List_With_Service.csv','w',newline='') as file:#creates a new csv file named 'Bikelistwithservice.csv'
                a=csv.writer(file,delimiter='')
                data=data
                a.writerows(data)#write each line of data