H1B extension

Hi My H1B visa is issued in May 2017 and it is expiring in jan 2018.
Is thier any problem in extending the same visa if I travel after Sept 2017
Is it mandatory to be in US for around 6 months to file extension.
Can we file extension from India without travelling to US?
I don’t have much clarity on the subject .
Please suggest.

Thanks,
Saurabh