Hi!
I guess there must be one or two messages on the forum depicting your situation.
You have two options :
- Either doing a RT to Colonia (taking the Buquebus boat from Puerto Madero) and you'll get 90 additional days automatically.
- Or you can go to "Migraciones" (in Retiro) but that's not recommended at all.
Eventually, to do a RT to Uruguay, you can also take a smaller boat from Tigre (you board the boat not far away from the Tigre train station). That's a bit cheaper (around 220 pesos the RT), a bit longer (2 h 30) but the journey is much nicer since you'll cross the delta through the many small islands (many things to look at : birds, nature, etc.). Using this method you'll arrive in Carmelo which is like Colonia (a little "dead" city) but with even less attractions (nice to breathe a different air though).
Last solution : don't renew your visa and pay a fine the day you leave.