Deactivate current object in array, activate next one?

I will consider you have a Page class array of, which extends Monobehaviour. If not, then i'd suggest doing so. Regarding your questions list:

1. You ideally should have an index of the current page, so create an int currentPage and use it.

2. Do array[currentPage].gameObject.SetActive(false);, or even better have a method on Page that does that and more if needed.

3. currentPage++, if you have sequential pages.

4. array[currentPage].gameObject.SetActive(true);

5. This has already been done on step 3.

Your question is a little general, but I will do my best to provide a decent example of how to accomplish this.

 private Page[] pages = new Page[numOfPages];
 private int currentPage;
 private float timeSinceLastTurn;
 
 void Start()
 {
      for (int i = 0; i < numOfPages; i++)
      {
            //Do whatever you need to initialize the pages
           pages[i].SetActive(false);
      }
      currentPage = 0;
      timeSinceLastTurn = 0.0f;
      pages[0].SetActive(true);
 }
 
 void Update()
 {
      timeSinceLastTurn += Time.deltaTime;
      if (Input.GetKeyDown(KeyCode.RightArrow) && timeSinceLastTurn >= 5.0f) // or whatever input you're using and delay you want between turns
      {
           if (currentPage < numOfPages - 1)
           {
                pages[currentPage].SetActive(false);
                currentPage++;
                pages[currentPage].SetActive(true);
                timeSinceLastTurn = 0.0f;
           }
      }

      //Repeat above code but inverted in order to handle turning back a page
 }
 

You will of course need to have some Page class defined derived from MonoBehaviour, but that part I'll leave up to you.