A person on a swing stands up. The time period of the swing:

The time period of a simple pendulum or swing is governed by the formula:
$$T=2\pi \sqrt{\frac{L}{g}}$$

where $L$ is the effective length from the point of suspension to the center of mass of the swinging body.

When the person stands up on the swing, their effective center of mass shifts upwards toward the pivot point, which decreases the effective length $L$ of the system. Since the time period is directly proportional to the square root of length ($T\propto \sqrt{L}$), a decrease in $L$ causes a decrease in the time period $T$.