Step 1:
Go to the Contacts tab and click on the contact where you need to add a property.
Step 2:
If the contact is Unassigned to any user, click +Assign and choose the new user from the dropdown menu.
The contact will now be assigned to the user that you chose. If you want to change the contacts assigned user, proceed to Step 3.
Step 3:
If the contact is already assigned to an user, click on the user profile icon and choose the new user from the dropdown menu.
The contact will now be assigned to the user that you chose.
To learn how to change the assigned user of multiple contacts at once, check out this article.