Move the camera when the mouse reaches the edge of the screen.

FollowCamera.cs

// Attach to the camera asset
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class FollowCamera : MonoBehaviour {
    private int screenWidth;
    private int screenHeight;

    public int speed = 5;
    public int distanceFromBoundary = 50;

	// Use this for initialization
	void Start () {
        screenWidth = Screen.width;
        screenHeight = Screen.height;
	}
	
	// Update is called once per frame
	void Update () {
        Vector3 position = transform.position;
        if (Input.mousePosition.x > screenWidth - distanceFromBoundary) {
            position.x += speed * Time.deltaTime; // move on +X axis
        }
        if (Input.mousePosition.x < 0 + distanceFromBoundary) {
            position.x -= speed * Time.deltaTime; // move on -X axis
        }
        if (Input.mousePosition.y > screenHeight - distanceFromBoundary) {
            position.z += speed * Time.deltaTime; // move on +Z axis
        }
        if (Input.mousePosition.y < 0 + distanceFromBoundary) {
            position.z -= speed * Time.deltaTime; // move on -Z axis
        }
        transform.position = position;
    }
}