When can I go outside of US after h1b approval?

My 2020 H1B is approved with Change of Status. I believe if i leave US before Oct1, my COS becomes invalid. What if i leave on Oct1. Will it impact COS?

No, it will not have impact. If it was not approved, then the COS will be abandoned.
As the H1B was approved and the I-94 COS will apply on october 1st, you are free to travel and re-enter before that using your current visa status. But, on October 1st, it will change to H1B. This was clarified by USCIS in a letter back in 2004.