Which region?
I swear people on this forum seem to think we can all read minds.
Grab the NUS downloader from
Github. Extract the archive somewhere, your desktop is fine.
US region:
0004001000021800 V5136
EUR Region:
0004001000022800 V5121
Tick Pack as CIA and click Download.
The cia for Streetpass Mii Plaza will appear in the same folder as the program. Rename it if you want.
Copy that file to your 3DS' SD card.
In FBI -> Titles -> Streetpass Mii Plaza -> Delete Title and Ticket.
Do this for both plaza files, if you have two.
Press B to get back to the main FBI menu. Select SD and install the cia you copied across earlier.
Use hshop (google that one) to install the updates and DLC for the plaza for your region. You can either use the 3DS app or the website to download the required files, copy them to your SD card and install them using FBI.
If you want a different version of Mii Plaza,
3dbrew has the details required. You can grab any version of any system apps this way without needing the whole firmware and something to extract the files from it.