All web applications may only have one system user at a time.
If you have not already created a new Web Application and would like to create a user:
On the Web Application screen, after clicking on Create Web App.
Locate User (Owner of this web application) and select the user.
You may select an existing system user or create a new one from this menu.
Or you may create a new system user from the web application screen.
If you have already created a Web Application and would like to change the existing user:
Navigate the menu to Tools.
Scroll down to Change Web Application Owner and click on Change Owner.
On the Change Owner screen, you may select an existing user or create a new one, as seen above.