How can I revert back to an editable Gantt Chart?

Hello,

I need someone's help here please. I was in the Microsoft Office Gantt schedule in my company's SharePoint and I tried to make some edits, however, I don't remember what button I clicked on in the toolbar and now when I click on Schedule it take me to image 1.

I'm able to get here, to this view of the schedule of image 2.

However, when I click on the 3 periods to change it to Gantt Chart here image 3.

It takes me to image number 4.

But now I cannot edit it, neither my boss, who is the owner of it and created the Gantt Chart.

Can someone please show me how to make it editable  and revert it back to where I was originally???

Thank you in advance.

-Norma C