You are accelerating when you change speed or direction.

You change speed when you slow down. So if you slow down at a red light, you are accelerating (often called deceleration or negative acceleration).

You change speed when you go faster. When you are merging onto a freeway and speed up to fit into the flow of traffic you are accelerating.

You accelerate when you turn a corner or go around a curve even if you speed remains the same.

is a change in velocity divided by time. Since velocity is speed with direction, a change is either speed or direction is called acceleration.