yesss i actually just did this last week! the hardest part was honestly finding the office itself. online it says it's 1355, which seems straightforward enough. but when you get there, you'll run into some misleading signs. like one of the street signs for Av. Antartida tells you that you're in the 1300 block, but that's not where you'll find the office, I think maybe it's an old sign? but basically if retiro is right behind you, you want to go right on Antartida, not left (which is what it would look like on google maps). you'll see a big Correo Argentino building in front of you, so go right of that, across the big intersection, and you'll find it there. worst case, you'll see a little entrance hut thing and the guy there was very helpful pointing me to the door I needed. once at the building, you need door #4, and you'll just wait in the main line. when you get the front (or someone might help you sooner depending on how busy it is) you just tell them you need to extend your visa, and they will give you a ticket with a letter and number. go wait in front of the window with that letter, and eventually they call your number to come up. bring photocopies of your passport! i didn't have anything and they were really nice about it, but there is a sign on the wall saying everyone needs photocopies of their documents, and if you don't have any you'll lose your turn and have to start over. but yeah then you just pay 300 and all done!
sorry for the excessively detailed reply haha but hope it helps, good luck : )
