B2 visa renewal for parents

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Hi,

My parents' B2 visa expires in January 2022. I would like to have them renew their visas before they can visit US next year. Is dropbox option available for visitor visa currently or should they schedule a visa appointment? Also, I see that only emergency appointments are available in Chennai consulate. So, I am not sure if a visitor visa renewal falls under the "emergency" category. Any input on this will be helpful!

Thanks!