Hi Manju,


The Process Builder is a new workflow tool that helps you easily automate your business processes by providing a powerful and user-friendly visual representation of your process as you build it. The Process Builder’s simple and powerful design allows you to:Create your processes using a convenient visual layout with point-and-click efficiency.
Create your whole process in one place rather than using multiple workflow rules.
Create processes by collaborating with different teams in your business.
Stop using Apex code to automate simple tasks.

Automated processes in the Process Builder consist of criteria that determine when to execute action groups and immediate and scheduled actions to execute when those criteria are met. Any change that causes a record to match the criteria can automatically trigger the action group.You can use the more powerful and flexible Process Builder to perform the same actions as workflow. The process builder doesn’t support outbound messages, but you can easily create one yourself with Apex. With the Process Builder, you can:Create a record
Update any related record—not just the record or its parent
Use a quick action to create a record, update a record, or log a call
Launch a flow—as an immediate or a scheduled action
Send an email
Post to Chatter
Submit for approvalIf you need your process to do more than what those actions allow, don’t worry. You can also call Apex from a process.


Please follow the below link for more information

http://salesforce.stackexchange.com/questions/60483/how-do-we-update-a-field-on-the-object-using-salesforce-lightning-process-builde

Best Regards
Naga Kiran