Drupal lets you create a new revision every time a node is updated. It allows you to track how the node has changed over time on your site.To enable this feature in Drupal 7, follow these steps belows

Step 1: On admin menu bar, click on Structure

Step 2: Choose Conten Types



Step 3: On content type choose Edit on each content type

Step 4:  Choose publishing options --> create new revision



   Save content type and DONE.

To view and manage revisions, use the Revisions tab when viewing a node (you must have "view revisions" or "administer nodes" permission to view revisions.) From the Revisions tab, you can view the individual revisions, revert to an earlier revision, and delete revisions (you must have the "administer nodes" or "revert revisions" and "delete revisions" permissions).