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You can walk in for the iEAD. However, there is a quota for every day, so I would suggest that you reach there very early in the morning, around 6.30 am or so. You would need your current EAD, receipt of EAD renewal and valid ID, prefrably passport.
If you do get in early, you should be out by around noon with the EAD. There are two entrances, and I think you need to go to the Orchard Street entrance. You can check their website, it gives good details.
Best of luck