Field updates allow you to automatically update a field value using workflow.
Open the workflow that you want to add a field update workflow action to, and click on the New button under Workflow Actions.
1. Select Field Update and click Add:
2. Now configure the field update:
Give the action a name (something that will easily tell a user what the action does)
Check the box for Time Based if required
Select the field that you want to update with a new value (this is not restricted to the Module that the workflow is based on). Once that is chosen, the options in which you can define will autopopulate. This could be:
A field with a check box - set to checked or unchecked
A free text field - specify some text to be added, or clear a field by leaving the value as blank
A picklist or a lookup field - set the value that you want to populate
Make the workflow action active by checking the box 'Make action active' and Save.