My h1b visa is approved and I will get my stamp in October. My question is if I can change my job till October? My employer didn't keep promises he gave to me and I want to quit my job I don't know if I can change my job till I get my stamp.

You must have at least two pay stubs in order to file a transfer, plus you cannot file for a job transfer before you enter US as H-1B for the first time.

I've been already working for this company and I am in US now. I work with my opt and I got my approval. Should I wait till October?