When a point charge is placed within an electric field, the...

Let the initial electric field be $\vec{E}_{0}$.

When a point charge $q$ is placed in this field, it produces its own electric field $\vec{E}_{q}$.

The net electric field at any nearby point is the vector sum of the initial electric field and the electric field due to the point charge: $\vec{E}_{net} = \vec{E}_{0} + \vec{E}_{q}$.

If $q$ is positive, $\vec{E}_{q}$ is directed radially outward. At points where $\vec{E}_{q}$ and $\vec{E}_{0}$ are in the same direction, the magnitude of the net electric field increases. At points where they are in opposite directions, the magnitude decreases. Thus, the electric field may increase if the charge is positive.

If $q$ is negative, $\vec{E}_{q}$ is directed radially inward. Similarly, at points where $\vec{E}_{q}$ and $\vec{E}_{0}$ are in opposite directions, the magnitude of the net electric field decreases. At points where they are in the same direction, the magnitude increases. Thus, the electric field may decrease if the charge is negative.