Visa Renewal/Stamping process from India

You have changed your employer recently but have valid (not expired) visa stamp on your passport then you are good with old visa stamp with new I797. If your stamp is expired and you are planning to travel then you may go with visa renewal process. You are eligible for drop box if you are applying for same class of visa.
Follow link for complete process
Step 1: Complete your DS160 online       DS160 FAQ
Step 2: Create your online account
Step 3: Submit your visa fee.
Step 4: Schedule an appointment date.
Steps 5: Interview – H1B Stamping Document 
Once you are done with your interview, track your passport