You don't need to. Okay, that's a bit of an exaggeration, but... The classic toaster sensed the temperature of the toast, and stopped when it was done. My family had one of these, and it was quite predictable. We never had to change the setting, and it handled a variety of kinds of bread.

Today's toasters, at least the cheaper ones, seem to be based solely on time, and so it's unknowable how a particular kind of bread will behave. You have to watch it and be prepared to stop it early.

Bagels were a problem if the hole lined up with the sensor. I don't think store-bought bagels were a thing in our area until long after the toaster was made.

There is probably a button, but at worst unplugging it works as toast is raised when the coils cool.

There is no button. You can sometimes make it abort by slamming the intensity dial all the way to the left. But yes, in general you have to unplug it, which is horribly inconvenient.

Turn the dial

But it’s both not very accessible as well as you’re resetting what you might want to keep as the default.