This option will let you select a user as the project Lead of a specific project/projects.
The Project Lead option of Bulk operations can also be used to replace project leads that are currently assigned.
You just need to follow these simple steps:
1. Filter through the projects you want to assign the project lead to.
2. Select Bulk Operations > Project lead.
3. On this first screen you can search and select the user you want as a project lead for this project/projects.
4. After choosing the user, by clicking 'Next' you'll be taken to the confirmation screen. Please, read carefully to see if the changes are what you want. 
5. Confirming this action, the process will start. As soon as it's done, you'll get to the confirmation screen, where you'll be able to see what projects have been changed.