In principle, you can stay until the date stamped in your passport. You don't need to inform anybody if your plans change.
In the last couple of years, I've twice ended up staying a day longer than I said I would, because of flights getting cancelled. This has caused me no problems whatsoever: it's not even been mentioned on subsequent visits to the US.
Minor changes to your itinerary shouldn't make much difference. If you said you were going to stay much less than 90 days and your new plan is still much less than 90 days, I can't see you having any problems. People's plans change and you have a definite reason for extending your visit. The potential problem is if you said you'd stay two weeks and you instead stay for ten. That starts to look more like your initial claim of two weeks was a lie, even if it wasn't.