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)