Hey I just bought an older co-op from the 50’s. I want to put a ceiling fan in but ran into trouble. I was trying to take down the existing box and it has a nipple in the middle with a flat head screw slot in the bottom. I bought the usual box and anchor mechanism to put in but I have no idea how to get this box out. Obviously I can't go in through the top due to the co-op situation. Please help. Maybe a YouTube video or something how to remove it. Thanks.
Jim Stewart's comment is pretty much right on.
That is not a nipple it is what is called a "hickey" by the National Electrical Code. It serves to attach the box to a hanger bar or other spanning device.
It looks like you may have a pretty stout box already and may not need the fan box.
I usually look for a ceiling hugger style fan and toggle bolt it directly to the drywall/plaster. This eliminates the need for fan box.