The default profile navigation tab is what users will see if they access their profile page or other member's profile page. In the example screenshot below, the default profile navigation tab was set as Timeline. Any users that will access a profile page (including their own) will first see the user's timeline. 

To change tab, kindly follow the steps below:

1. Navigate to the WordPress Admin > Appearance > Customize

2. It should take you to the frontend of your side and there will be a Customize sidebar > click on BuddyBoss Platform

3. Click on Profile Navigation

4. Select your preferred default navigation tab under Profile navigation order

5. click Publish to save the changes.

Please see the sample result below when the Photos tab was set as the default navigation tab: