H1 petition about to expire,how to extend it

My H1 petition is about to expire on 31st July 2019, even stamping slots are full till August.Could you please advise me how can I increase petition validity before it gets expired/stamped

It is not just stamping, you need to find an employer to file H1B transfer or ask your employer to file H1B extension with USCIS, then you need to go for stamping. ReadH1B Transfer Process Details