I have a set of over 400 days within my time series, my goal is to detect when a particular trend breaks and another one begins. For example, for days 1-30, an ARIMA(0, 1, 1) will be the most appropriate, but from days 30-74 an ARIMA(2, 1, 2) will the best fit.
Is there a way I can check something like this? My understanding was the ACF function but I'm a bit confused about this now too. Could someone point me to some resources that may explain?