how to delete inbox

Do you mean you want to delete the submissions in your inbox? Or? 

Can you please elaborate more about this?

If that is what you meant then those are actually tied to the submissions you receive. To delete your submissions you need to go into your submissions viewer and either delete them one at a time  or alternatively you can delete all.  However, be advised your submission data cannot be recovered so be sure to make a backup export if you plan to refer to any of it later on down the road.