Sustain a Spell has been changed broken into the Sustain action and the Dismiss action.
Choose one of your effects that has a sustained duration or lists a special benefit when you Sustain it. Most such effects come from spells or magic item activations. If the effect has a sustained duration, its duration extends until the end of your next turn. (Sustaining more than once in the same turn doesn’t extend the duration to subsequent turns.) If an ability can be sustained but doesn’t list how long, it can be sustained up to 10 minutes. An effect might list an additional benefit that occurs if you Sustain it, and this can even appear on effects that don’t have a sustained duration. If the effect has both a special benefit and a sustained duration, your Sustain action extends the duration as well as having the special benefit. If your Sustain action is disrupted, the ability ends.
Player Core pg 419
You end an effect that states you can Dismiss it. Dismissing ends the entire effect unless noted otherwise.
Player Core 419