Change font atlas of font asset (TextMeshPro)

1)I need to export font atlas to modify it in Photoshop
2)Then I need to change original font asset to modified
Any ideas?

Unsure if this is still an issue but for anyone else googling :

Right click TMP font asset and click export atlas. Make any changes that you need to make.

(Unsure if this is necessary) View the font asset in Debug Mode (3 dots icon top right of inspector) and you can replace the Atlas texture (but nothing will change at this point).

Select the font material, open the Debug Settings at the bottom, and drag your modified texture into the atlas texture slot.