To modify a pre existing sub nav of a profile tab, you must first create a tab entry and mark it as modifying existing tab.
Once you have done that, you will see the options like below in the sub nav meta box.
To modify a pre-existing sub nav, please enable the “Is Existing” checkbox.
All these settings are optional except the slug. The slug is used to identify the sub nav. To find the slug, you should look at the url. The slug comes after tab slug as shown below.
In the above example, we have renamed the “Requests” to “Friendship Request” and moved it as the first item as shown below.
Complete demo for how to modify ore existing sub nav items
To add a new subnav item to the existing profile tab, please follow this guide.