f.write('\n'.join(newlines))        filename = '' # туты имя файла пишешь
with open(filename, 'r', encoding='utf8') as f:
    lines = f.readlines()
newlines = []
for data in lines:
    data = data.split(';')
    data[8] = data[8].replace(',', '.')
    data = ';'.join(data)
    newlines.append(data)
with open(filename, 'w') as f:
    f.write(newlines)