Change the Default Tab Viewable When a Record is Opened in Salesforce Lightning Experience

To change the default Tab that is viewable when a record is opened for any Object in Lightning Experience, follow the below steps:
 

1. Go to Setup > Object Manager (Tab).

2. Select the object related to the record.

3. Once the object page opens, select Lightning Record Pages in the left column and choose the page you want to update. When the page opens,  select Edit.  The Lightning App Builder will open.

4. In the Components column, select Tabs or click on the tabs section of the page. The "Tabs" properties will open in the far right column.

5. Looking at the "Tabs" properties on the far right of the page, you will see ‌the "Default Tab" box, and this is where you select the default Tab that will be viewable when a record is opened.  Select an option in the drop-down menu for the new default tab.

6. Click Save and then Activation.

7. A window will display showing your current defaults in Lightning for this page. You can just click on Close or modify the defaults.

8. Once the Defaults page is closed, click on the back arrow at the top left of the page to go back to the Object Manager.

Follow these same steps for any other Object Lightning record page where you need to update the default tab that is viewable when that objects records are opened.