do not disturb

If using iPhone X or newer, swipe down from the right side of the screen. Tap the moon shaped Focus icon, then tap "Do Not Disturb". This will toggle the setting, so you can turn it back on the same way. For iPhones before the X, swipe up from the bottom of the screen.

I'm assuming you don't want to change your scheduled "Do Not Disturb" times and just on occasion want to turn it on or off by overriding the scheduled time.

Turn it off using the Focus function in control center

Apple - Support - Search