Maybe I am missing something obvious, but in Attribute Manager (ArchiCAD21 and before) I can't do anything other than renaming layer combination, not actually set visibilty, lock/unlock, intersection group of layers in layer combination. Can change those in the Layers panel, but that won't change a layer combination.
It is great for importing layer combinations from other projects / attribute files though!
Just had a look around. In AC22, with the combined Layers / Layer Combination page, you can select a Layer Combination and the Layers change to match, which can then be modified. In AC21, between the two pages, I cannot find any way to select a Layer Combination to modify on the Layers page. The Layers page stays as which ever Layer Combination was active when the Attribute Manager was opened and the active Layer Combination becomes "Custom" when modifications are applied.
AC22-23 AUS 7000 Self-taught, bend it till it breaks.