I remember doing some menu changes on my 3DS some years back.
I used this software called MSBT Editor Reloaded, though the creator released Kuriimu, which he claims is more advanced.
I can't remember exactly how to edit the system UI, but this is a good place to start.
https://gbatemp.net/threads/release-msbt-editor-reloaded.406208/
https://gbatemp.net/threads/release...olkit-for-authors-of-fan-translations.452375/

Also, welcome to GBAtemp!

 

https://gbatemp.net/threads/tutorial-edit-the-homemenu.424449/
I believed I followed this tutorial, however, I don't know how to edit integrated 3DS software.
https://gbatemp.net/threads/tutorial-edit-the-homemenu.424449/