H1B Visa Extension

My H1b got approved last year but it was approved for only one year and the validity is till Sep end 2017, currently I am in US and I would like to know that what is the procedure to extend my H1b visa before expiry? I am not sure why the H1b got approved for only 1 year and if I file the extension what duration the my visa may get extended? will it be for 2 years or only 1 year?