described how to edit “font atlas”.
There you can export “font atlas”, edit graphics and put it back.
I was able to do the same with TextMesh Pro 2.0.1, but there is no option to put this “custom font atlas” back.
I achieved desired behavior by putting “custom font atlas” into Material/DebugSettings/FontAtlas.
The problems is “font asset” refers to “font atlas” asset, while using “custom font atlas”.
Is it as designed?
The font asset contains 2 sub objects. These 2 sub objects are the default material and the atlas texture which is referenced by the material. Since these are sub assets, they cannot be replaced or removed. However, you can assign your newly edited / modified atlas texture to the default material of the font asset and that will work.