Useful sources to edit u8g2 fonts:

bdf font files for u8g2 font library:
https://github.com/olikraus/u8g2/tree/master/tools/font/bdf

Tool to edit bdf files:
https://github.com/olikraus/u8g2/tree/master/tools/font/fony

Tool to convert bdf font files to u8g2 source code:
https://github.com/olikraus/u8g2/tree/master/tools/font/bdfconv