apply for H1 visa in China or

or mexico before leaving for China.

You have to apply for H1 visa outside of US. So either you go back to China then apply for the visa there, or go to either Canada or Mexico and apply for the H1 visa there first before flying back to China. Keeping in mind, that if you are denied a visa in Canada or Mexico, then you are technically NOT allowed back into US, but in practice, many do make it back.

There is no such thing as applying for H1visa in US. H1 revalidation was cancelled last year and you don't qualify for it anyway, because it is only for people who already have H1 visa stamps on their passports.