You need to expand the section for the text style.
You then choose what part of the schedule you want to apply the change to and then change the text style settings.
But if you are in a teamwork file, you may need to reserve the schedule or scheme settings so you have permission to adjust them.
If you can not expand the text style section at all, what version of Archicad and what Operating System are you running?
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10