Activating Lightning Record Pages Manually

You can manually activate a Lightning record page and assign it as your org's default page as follows:

 

1. From Setup navigate to Object Manager.
2. Click the object label you want to activate the page for.
3. Click Lightning Record Pages.
4. Click the Lightning page you want to activate.
5. Click View, the Lighting App Builder opens.
6. Click Activation.
7. Make sure the Org Default tab is selected.
8. Click Assign as Org Default.
9. Select the form factors you want the page to be available for.
10. Click Next.
11. Check the assignments are correct and click Save.

For more information, search for "Activate Lightning Experience Record Pages” in the Salesforce Help.