Currently BFFNT supports Wii U and Switch. 3DS support for both bffnt, and bcfnt is not supported yet but will be at a later date. Only images can be edited, however that should be enough unless you need custom glyphics, which is currently unsupported at this time.
Open your BFFNT file. You can adjust the image being displayed in that dropdown list.
The options on the left and right side you can ignore, as they are unnecessary for editing atm.
Right click on the texture you want to edit, and hit "Export".
Then for editing, you want to make sure all characters align with the original. Then flip the image upside down and save it. Font images are always upside down, but displayed rightside up in the tool.
Now save your image. If you use a DDS image, make sure it has only one mip map! The format also should be the same to prevent possible corruption (will vary depending on the game).
Right click on the texture you want to edit, and hit "Replace".
If you did not use a DDS image, a settings window will appear, simply hit OK as default settings are fine.
Now simply save your file and now it's replaced!