urgent help needed

Sorry I cannot type Chinese at work

I am currently on H1 non-cap visa. I was planning on starting working for company A on Cot 1st, who has obtained H1-cap visa for me. Due my very recent knowledge that company A does not sponsor GC, I have decided to accept an offer from company B. My question is, do I have to ask company A to withdraw their H1 petition. Or can I simply ask company B to set the start date just a few days later than Oct 1st. What do I need to do in this sort of situation?

Thank you all very much for any help and suggestions in advance.