Any changes you do in a feed plan or on fixed feed amounts on cow level, will have effect after a feed start. There are 3 ways to do a feed start:
Automatic at the set feed start time. Go to Settings > Feeding - Feed settings > Feed start to see when the automatic feed start is done.
Manual feed start for all cows. Go to Settings > Feeding - Feed settings.
Manual feed start for one or more specific cows or production groups. Go to Quick entry > Selection > Next > Action 1 - Feeding - Manual feed start.
A manual feed start is a deliberate action by a user and it does not take the limits of a feed plan into account. Keep this in mind to prevent cows from suddenly getting too much or too little feed.