An Owner has to be a Group (either the Company or a Group/team). You can, however, edit the Lead (an individual).
You can do this in two ways:
1. Directly from the Objective modal by clicking on the Lead name:
2. By clicking the 3-dot menu beside the Objective's name (on a profile page) > Edit Objective > Lead:
Have a question? Contact our friendly Support team via the in-app chat at the bottom-right of your screen, or email email@example.com.