Follow the steps below to enable field history tracking:
  1. At the top-right corner of your screen, click Setup (or click your name > select Setup)
  2. Under App Setup, expand Create and click Objects
  3. Click the object that contains your desired field
  4. Under Custom Fields & Relationships, click Set History Tracking
  5. Mark the checkbox next to the field you would like to enable field history tracking
  6. Click Save
Note: The history that is logged will begin the time the field is enabled. Any changes that took place prior to enabling this will not show in the history.