I have been playing Streetpass Mii Plaza for awhile now, I have found 2 nice hats I want but I can't get them, is it possible I could use the settings to go back for them?
You don't unlock hats when receiving streetpasses from random users.
You can obtain some by purchasing Mii Force, Flower Town, Warrior's Way, Monster Manor, Ultimate Angler, and Battleground Z, or by completing achievements in said games and earning Plaza Tickets. You can also earn Plaza Tickets by collecting birthdays with StreetPass Birthdays. Other outfits are won while playing Find Mii I & II.
You can find the complete list here.