Hello! I am trying to add custom graphics to bitmap font asset but Atlas is not changing!

Hi all! I am trying to reproduce what is explained in this video:

Youtube Video

I know this video is made years ago, so can someone tell me what is changed? I can not set or modify the Atlas of the font asset, I can only set the material (in another thread I found an answer which says now I have to set the atlas I modified as material of the Font asset, I tried but in the glyphs table I did'nt found the image I added by the way).

In this video he just saves the png file and it seems the atlas is automatically updated. Can someone of you suggests me a workaround for this?

thanks in advance