First your new employer need file petition for you. You get offer letter from them and then they file Form I-129. After that, you can start work with the new employer once USCIS approves it. Usually takes 2 to 3 months.

Process can take one to four months. To transfer h1B visa to a new employer, it goes like this:

1. First, you get a job offer.
2. Your employer will file LCA with Department of Labor.
3. Your employer will send h1b form to USCIS.
4. Then wait for USCIS to say yes or no.