By default, you can directly edit the contents in the cell. Double-clicking the cell moving the cursor in the cell, you delete, modify or add more contents. If you do not want people to edit directly the contents from the cell, you can setup disabling editing directly from cells. Please following steps below:
Step 1: Click the "File" Tab from the ribbon;
Step 2: Click "Options" from the left navigation menu;
Step 3: Click "Advanced" from the left navigation menu in the dialog box;
Step 4: Uncheck "Allow editing directly in cells" under the section of "Editing options";
Step 5: Click "OK" at the bottom.