Things you will need:
Kuriimu: https://github.com/IcySon55/Kuriimu/releases
and of course a romfs of nsmb2.

Tutorial:
Step 1: Once Kuriimu has downloaded, extract it and open the 'Kukkii' application file.
Step 2: Once Kukkii is open, you will need to hover over 'file' then press open.
https://gyazo.com/31f9ec83ed1a39a5f3477222c1538226
Step 3: a file manager window will pop up, what you want to do is navigate to your romfs, and go to Model/Demo/ and then select 'title_logo' (if you are playing gold edition it will be 'title_logo_ge' instead.)
Step 4: You will now have 3 (4 if you're on gold edition) pictures like this:
https://gyazo.com/c33ac835b4b34635be389fd062c0357f
(Mine looks different as I have already edited my logo.) Now you want to click the 'import image' button on the top left and replace the 3-4 files with your logo.
Step 5: Once you are done, press the save button on the top left.
You're done! Hooray! I hope you have found this tutorial helpful, give me feedback if you have any issues. This is my first tutorial on this game so tell me if its alright!

I have quite a few problems with ohana3ds, this tutorial is probably simpler.